Originally Eredar lords corrupted Sargeras when he destroyed them, but that was later retconned.
Draenei are a major retcon. They were originally their own ‘shamanistic race’ but were later retconned during BC to just be uncorrupted Eredar. Also a missed opportunity to force the player to be broken and work their way up to being pretty but whatever.
With this in mind, the crystal quest driving the owlkin mad in darkshore is a technical retcon as a result of BC. It also kinda doesn’t make sense since it would imply the Draenei were already there during Vanilla.
Garona being half-Draenei is a retcon since she was originally half human. The reasoning is that they changed the time of the first war and basically made it so it was impossible for her to be half-human. But then said she was half draenei…then were like ok well her human appearance? Yeah that was because she got corrupted. This is probably the best example of why people don’t like retcons. Because it really makes so very little sense.
Worgen are not really a ‘retcon’ 100%, but the ones in Vanilla cast shadow magic spells and whatnot meanwhile the ‘current Lore’ says they’re just druids from the emerald dream that lost themselves to some sorta rage. That doesn’t explain why they’re casting shadow spells but whatever.
I have read that night elves being allies with the creatures of Feralas was retconned and they’re just ‘neutral’ now. I’m not sure if this was a vanilla change or not.
The player killing Onyxia is a retcon. In current Lore Varian killed her (the first time).
Edit: Also Nefarion not being killed during Vanilla.
Originally I THINK all the Blood elves followed Kael’thas to outland so none left on Azeroth. They weren’t called Blood elves till later either. So any Azeroth elves wouldn’t just be Blood elves. They changed it later so they could have horde Belves. So technically right now, there would be no Blood elves in this version of the game. Just high elves.
The War of the ancients in general has been blown up and reconfigured a whole crap load due to time travel garbage to the point I wouldn’t know exactly where to start with it and how it would affect classic.
Not sure if this is a retcon of lore during Vanilla but I believe Aegwynn’s motivations and general personality got retconned from her being an arrogant, self serving witch to something more ‘heroic’ and I use that term loosely. I think the original book was titled something like the last guardian or something like that.
WoD basically throws out so many questions about the lore in general that I’m not even going to bother trying to figure out what’s a retcon and what isn’t. I really don’t care. I really don’t.
There’s actually a few other things, but some of them I’m not really sure are retcons after vanilla or as a result of it.
There are quite a few pages out there about retcons, but I don’t think there are any dedicated to what comes after Vanilla specifically. You kinda have to piece it together yourself.
Edit: Also my favorite retcon is this https://i.imgur.com/Ds9lKf5.png (note, not an actual retcon, just a silly picture/story)