So I was wondering what your opinion is for scripted RNG when it is important to the plot, like someone drawing a specific card in a deck of many things, or tooth in teeth of Dalver-nar. Or saving throws with a spell that would be important to plot. This is not for damage rolls or anything, but plot stuff.

I'm confused. If it's Scripted so that the result is predetermined, how is it Randomly Generated? 

If it's that imported to the plot I would recommend you just narrate an "unskippable cutscene". 

Well it would be something that is normally random

I think what you are referring to is dice fudging by the DM when they use hidden rolls to obtain a result that they want to see rather than the one dictated by the dice.

DMs can use the technique by having a killing blow miss a target or they can have an NPC either fail or pass a critical saving throw. Some DMs use it as a technique to get the game to fit a narrative that they have in mind before the encounter starts or they sometimes do it to presumably heighten tension by trying to tailor the difficulty of the encounter by controlling the die results of the NPCs and then presenting those results to the players as random.

Some DMs do it to reduce the capabilities of spell casters at higher levels where spells like Hypnotic Pattern or Banishment can take strong opponents out of a fight. A deadly encounter can become trivial if it is split into two parts by a cleric banishing a couple of the most challenging opponents ... on the other hand, if the DM makes the encounter extra deadly and banishment fails then it can turn into a TPK. Similarly, if they intentionally made the encounter more challenging then the DM simply has the NPCs fail the saving throw. However, this is assuming that the players do exactly what the DM wants them to and then the DM adjusts things so that even when the players don't follow the script, the DM attempts to manipulate the dice to get as close to their intended script as possible.

Heavy use of dice fudging eventually becomes obvious to the players and when they realize that the DM is either intentionally killing them or alternatively making sure they survive by "faking" the dice rolls then the player enjoyment will typically go down unless they are only playing for the "story" (which I find is a minority of players).

In any case, there is no such thing as a "scripted RNG" ... there is only the DM dice fudging or not.

Whether a DM should fudge or not is controversial. Some DMs do so. The current trend appears to be that DMs roll openly in front of the players so that dice fudging isn't an issue. Instead, the DM can adjust NPC hit points and abilities a bit but the players know all the die rolls which can add a sense of tension around almost every die roll especially the important ones.

Personally, I roll in the open these days and don't fudge die rolls but that decision is up to every DM.

Scripted isn't RNG by definition. But just about every DM there is fudges rolls occasionally, some just do it more than others. And some are better at lying about it

One of the most low-key important skills for a DM to get the hang of is knowing when to call for a roll and when to narrate. I try only to initiate a roll if the outcome is changable, and if failure wouldn't just halt the action. 

For example, I once played with a DM who had us roll for EVERYTHING. We were crossing an invisible bridge so we had to roll Investigation to find it, Perception to know where the edges were, and Athletics to actively walk across it despite not being able to see the surface. Nobody succeeded on any of the rolls, even though the DM described other people walking along the bridge that we could see, but we kept falling off and going prone and slipping into the water. The DM seemed to be having a lot of fun, but nothing else was accomplished for most of that session. "The Players v The Invisible Bridge" is not an encounter I remember in a fun and exciting way. It was tedious, and many of the problems caused by the bridge's invisibility could easily have been solved without rolls. We could have used a pole to tap along the riverbank until we found the surface and continued to use it to cross. We could've used the paths of the other people on the bridge to identify where it was safe to walk. We could've done anything, but anytime we tried to describe any problem solving, they just insisted we roll.

This works the other way around too. If you NEED something to go off a certain way for your plot to happen right, then you can just not roll for it. Narrate. Though I would try to keep this kind of thing to a minimum because you shouldn't let the plot you have in your head become so cemented that it erases the potential for player choice.

TLDR; if you need to fake RNG, then you don't need a roll. If you roll, then the outcome of the roll should stand.

As others said scripted and random are opposites.

But to answer what I think the question is getting after, the important thing is to never put plot-critical events behind a roll. Sooner or later, the dice will fail you. If you need it to happen, just have it happen, don't roll for it. I'd also add, I would never do something like this "on-camera" If the PCs are there, they should have a chance to impact the event, or there's no point in them being there. In that case, the dice may be bad, and the characters may fail (or succeed, or generally do something you don't want them to do) and then you just need to deal with the fallout. Just how sometimes the character's plans don't work, sometimes the DM's plans don't work.

Yeah this is a cutscene and they can be very useful. Obviously you don't want to do it too often, but if your plot falls apart - or just becomes a lot less interesting - if a certain action fails, a cutscene avoids that. I think its fine if you communicate clearly and your players understand that you're telling a story together as opposed to just being random agents of chaos focused on sabotaging your attempts to build a narrative.

Just keep it separate from regular play. Cutscene at the end of a battle to have your BBEG escape, but don't cutscene in the middle of combat just to let him live another round and use his cool power you wrote. It's best to be very clear about when your players have agency and when they don't.

Now if you want to have a cutscene with a spell doing something to the party, I would not use a normal spell that would normally grant a saving throw. You describe the effects without invoking the name of a spell they know would usually grant a saving throw. Instead of Banishment, they are shoved into a swirling portal. It's not "something that would usually have RNG," it's a different effect entirely. DMs are not limited to the PC spell list.

I agree with the people saying not to pretend something is random when it's predetermined.

I also want to add: Don't put the PCs in a situation where it looks like they should be able to influence events when they can't. Because they're going to want to try, and it feels bad when their best tricks and great rolls run smack into a wall of narrative fiat. You don't have to make it easy, but if you give them the appearance of agency, they should have a chance. 

You can get away with the occasional scene where you tell them outright "You are out of your depth here; this is not something you can influence", or "If you fight here, you will die", but these are best kept to a minimum, and when possible give them something meaningful they can do. Maybe they can't stop the dragon torching the city, but they can get people to safety.

Random events purely involving NPCs are different. If the king pulls from the deck of many things, you can totally fix that roll. Similarly, the assassin doesn't have to roll to hit to stab the king, and the king doesn't get to make his save. (After that, whatever you expected to happen will go completely differently from how you expected, but the setup is legit.)