Ok in The Wild Beyond the Witchlight book it introduces fairy's as a playable race which is super cool. I do have a couple questions about them though, as they are fey and not humanoid, would they be forced to follow rules that other fey might such as not being able to knowingly tell a lie? I know there are other things like cold-iron damaging them but I'm not too focused about that. If I couldn't lie however would be very interesting and fun to play with. What about the Fey's ability to take or receive intangible things like emotions or even someone's thanks (saw that on reddit somewhere). Would you as a Dm allow something like that where if an Npc or even another Pc make a deal with a fairy (npc or pc) and it would compel both sides to keep their end of the deal (I'm pretty sure Fey in general also try to keep their deals fair). What about true names? 

I’m pretty sure you’re bringing in some house rules and rules from other editions and possibly some lore from outside D&D. For example, cold iron used to be in previous editions, but is not a thing in 5e.
And while there may be stories and myths about how fairies behave across the world (such as not lying), D&D creates its own lore. That is often informed by real-world mythology, but it has become it’s own thing.  

So, I’m going to give the standard response of: rules only do what they say. Since there is nothing in the fairy racial description that imposes these qualities, they don’t exist.

Now, there are certain spells and powers that only effect humanoids or fey. (Animate dead, for example, targets a humanoid. Ancients paladins can turn fey like they are undead. I’m sure there’s other things) That’s something to watch out for, but otherwise being a fey, rules-wise, isn’t all that different from other PC races. Now, if you want to say your particular fairy can’t lie, go for it, sounds like it might be fun to play. But the game is not forcing that upon you.

Fwiw, there are other fey type creatures that can be a PC, such as satyrs and centaurs. None of the special things you mention apply to them either.

I have a question, as elves have fey ancestry, which does give them extra rules, would these rules apply to Feys? to me it seams weird that an elf, who is related to feys get advantage on saving throws against being charmed and magic can't put them to sleep, but a full fey doesn't get these traits!!

Again, no. They only do what the books say they do. Nothing more, nothing less.

It could be the elves have been so far removed from their fey ancestry that they developed this new trait, and other fey traits fell away. Like in a Darwinism sort of way.

Of course, the real reason is game balance, and that when the designers wrote that racial trait for elves, they had no idea that years later they’d make a book full of fey creatures which would cause a discrepancy. 

One thing that struck me as strange was the lack of reference to wearing metal other than to the restriction of fairies' flight when wearing medium or heavy armour.  This is in reference to a wide range of fey creatures that don't wear metal armour but, of course, this might be because metal may be somewhat lacking in places like the feywild.

Looking at the creatures in wbtw this generally confirmed this view except for the strikingly different situation of the plate wearing Bullywug Knight.

For me the image of fey is of creatures that generally avoid metals and technology or at least that typically have no attraction to it.  It's on this point that it seemed strange at least to me to find synergy between the harengon and the artificer's armourer subclass, namely that a harengon that gains the Thunder Gauntlets ability so as to give opponents disadvantage when attacking anyone else and then, as bonus action, rabbit hop away just leaving the disadvantage.  It's a nice combo.  I just think it's a shame that it's come in a way that brings armour and fey together and that seems at odds with wotc's promoted images of harengon.