Are legion , BFA, shadowlands and onwards are accountwide or no?
I would guess Not… At least in BFA, Legion, and Shadowlands. Blizzard didn’t do a lot of “Account Wide” stuff back then.
Nope as Tiannah says those expansions predate the warband system so any raid skips is on a per character basis
They are in fact account wide.
Source: Me on my many alts during Collector’s Bounty.
Edit: and have been for quite some time since Dragonflight for BfA and prior raids.
Once the quest is completed the skips are available account wide.
None of those expansions AFAIK has an account wide quest - though WoD HFC quest is account wide (as in every character can contribute to progress).
BFA has some oddities - some don’t have a skip (Eternal Palace) at all and some don’t have a quest, just a requirement to get the mythic achievement (Battle of Dazar’alor).