Is there a 5th ed equivalent to 2nd Ed deities and Demigods?  Looking for official information on the various pantheons.

Not officially, no.

SCAG has some info. on the Faerunian deities. A few of the other books have info. scattered about. But no single specific book, no.

That long ago book did something weird to the game.  Players were supposed to use the book to pick things to worship and interact with, but it got treated like another Monster Manual instead for high end games.  Who doesn't want to ride Oden's horse, while using his spear or switch up and use Thor's hammer? That's the problem with giving powerful things stats, and that's why very few deities like Oden are given stats.  There are some that exist.  Do you really have players that can kill Tiamat?  Do you want them to have that kind of power?  Do you want her removed from the game?  Same with Lolth.  I'm not sure if there are current stats for her, but the entire Drow race revolves around her.

Right, they don't stat out deities anymore because the gods really are not supposed to be something you can fight, even at 20th level.

But some players *want* to fight gods, and for that you need stats. I think you can probably find some ideas online or adapt the old book yourself, but anything you find will basically be homebrew, as there is not and probably won't ever be any official stat block for deities.

The only gods currently published are either weakened manifestations (The frostmaiden) or avatars (Tiamat) and killing either of them do not actually kill the god in question.  I assume it will never get more than that in 5e.

Personally, I don't see the difference between demi-gods and the super powerful creatures we already have (archdevils, demon lords, empyreans, etc...) so I would consider those to already exist in the game, or at least be "statblock-able".  I'm by no means a lore-expert (nor do i really care to be, I usually run at least partial homebrew worlds now), so there may be a meaningful difference between demi-gods and these creatures "officially", but I wouldn't consider there to be one.

:-)  So far as it goes, we get the term "demigod" comes from Greek mythology.  The child of a true god, and a moral, "half god" or demigod.  That's what Hercules was.  It's not all that hard to be on the same level as he was.  It's around Tier 4.  Add an Epic Boon at level 20, and a couple of Artifacts and you're good to go.  You can even give your player character demigod a Hammer of Thunderbolts, a belt  of Giant Strength, and some Gauntlets of Ogre Power.

The things listed in the Deities and Demigods book were very weak.  Player characters could and did defeat them, and the items they got from that were impressive, they weren't; all that awesome.  Later editions suggested that what you fought were only Avatars of the gods, and those, when defeated, returned to the home plane.  That's what Tiamat is.  They said that the True Gods didn't have any stats, and couldn't be affected by mortals in any way.

So how then do you define the power of a True god?  In my own campaign, gods are the greatest of all spirits, and they spawn out of Chaos, then are shaped by the forces of the other Alignments.  Where do they form?  You might think it would be Limbo, the plane of pure Chaos, but I say they show up in the Outlands.  The forces of Alignment kind of gently push them into the Outer planes from there.  I tried to work up the stats for a True god, but I really couldn't do it.  I was limiting myself too much and trying to extrapolate from the stuff in the Player's Handbook and such.  It wasn't at all satisfactory.  I gave them only two Epic Boons, but a True gods is someone who can give them out.  So they'd have all of them they liked.

I tried to define a class for them, but again, that's not enough.  They'd be able to make their own unique "class" by grabbing any ability from any class they liked ala carte.  They wouldn't have any of the limitations that classes apply.  Their spells would all be cast at will with no components an unlimited time per day, and they'd have all the Feats they wanted.  Hephestos would be making their armor and weapons, he's the divine armor and weapon smith so he'd need everything an Artificer can do, everything a Fighter can do, and who knows what more.

So back to Tiamat.  You'll find her Avatar in Avernus.  You'll have to defeat the hoards of Hell itself to reach her, you'll probably have to deal with Zariel on the way.  You may get the ire of Asmodeus, and you'll need to defeat all those Ancient Dragons she keeps as minions.  Once you have destroyed her Avatar, then you have to go to the Outlands, and she could be anywhere in it.  You track her down, and now all the True gods may be watching and could take an interest.  If you're willing to deal with celestial politics and the chance you might have to face the agents of the True gods like Solars, and the True gods themselves, then you can actually try to destroy Tiamat.  She'll have all her scores at about 35 or better, probably 40 in whatever she needs for her class.  She probably won't need weapons or armor, but there are items that could be somewhat useful like the One Ring, but more powerful.

Keep in mind, she's a native of the Outlands, she can travel to any other plane she likes in an instant, so keeping her there long enough to destroy her is going to be interesting.

That's what a True god might be like.  In my own game there's something beyond all the gods, and that's me.  I don't think they can be defeated no matter what, and what I've suggested in only a rough approximation.  :-)

The things listed in the Deities and Demigods book were very weak.  Player characters could and did defeat them

Were these beings "very weak?" Or were they just weak compared to the abusive practices many campaigns (I'll be honest, including mine) used that utterly broke the game if you continued to the upper levels?

For example, one house rule we had was that we moved everyone's die up one class for hit dice. So Wizards, supposed to be d4, were d6 in our game, and fighters, supposed to be d10, got d12. We also started with max on the first level hit die, and we re-rolled 1s. These rules will result in PCs at higher levels who are significantly more powerful than the rules expect. A 10th level fighter, for instance, could expect to "roll" 12 + (16+96)/2 + 3 = 71 for hit points, and since we also allowed much more generous ability score rolls, was probably looking at +30 from CON, or 101 hp. A fighter rolled under the AD&D RAW would have been (9+90)/2 +3 = 53 bas hit points plus probably 20 from CON, or 73. Thus, just from this house rule alone, our characters had on the order of 25% more hp than they should have by RAW.  We also cheated/broke the rules for encumbrance, for weapon-vs-AC tables, and other stuff.

The only reason this overpowered nonsense (which, make no mistake, it absolutely was) never bit us in the behind was we usually stopped playing after a few levels, switched to Champions or something, and when we came back, our group had shuffled a little and we started over at level 1 with a new party. If we had gone through to the high level game when you would face gods, we might have beaten them too, but it wouldn't really be legit, because we cheated.

I'm not accusing anyone on this thread of anything, but just telling my own story, of how my group bent the rules past the point of breaking in many ways to make our characters way OP compared to AD&D RAW's expectations, so it's hard to agree with the claim that these gods were underpowered. After all, the gods all have far more levels than any fairly, properly played party could possibly have without cheating. Apollo is a 20 cleric/14 druid/15 ranger/15 thief/23 bard. Non-human characters can't even reach most of those levels (unless, again, like we did, you cheat and lift the race-based class level restrictions), and some of these (druid and bard) are max level. Apollo is the equivalent of an 87th level character. What D&D group ever adventured long enough to get to 87th level? He has every druid spell in the game, all the bard abilities, all the thief abilities basically maxed, all the ranger abilities, and all the clerical abilities. He can cast every druid and cleric spell in the entire game. Multiple times. And then on top of that he has his own special abilities.  His lyre alone can cast bestow curse AND disintegrate once per round. He has +5 to all saving throws, 56% magic resistance, and a +4 bow that shoots +3 arrows. I don't see how that is weak, compared to how RAW characters are *supposed* to be built and played in AD&D.

Dragon magazine used to do April Fools things for the month in question, and some of them were delightful.  One of them was that the editors told about some of the things people had written to them and complained about.  One was about a guy who they called "Wardorf" He was a level 200 Magic User, who had taken over the entire world of Oerth, where he ruled from the city of Greyhawk, and he demanded that all setting material published from then on would include his name.

I got a kick out of The deities and Demigods book when I was a kid. I don't think we tried to come up with ways to defeat them. We did do a "choose your champion" thing wherein we chose various gods and had them fight each other.

The thing most folk forget is the extra material like lore, magic items, and creatures.

I would not have any party be in a place to challenge any god directly.