Unavailable Content Per Expansion

Hi all! So I'm returning to WoW. I'm a diagnosed completionist. I know for sure (and accept) I can't do everything as some stuff has been phased out or were time-sensitive events, etc.

I just started a new character and plan to go zone-by-zone and do everything available. I know how that sounds to some, but ignore that. :)

What I was wondering, is there a list of things that are no longer obtainable due to the "next" expansion coming out? Is there anything that I have to do before moving on to a piece of content because of phasing or lockouts?

I just want to plan ahead a tad before moving on to the next chunk.

Thanks for your help folks!