Many of the earliest Marvel adventures of the likes of Captain America have been reimagined and rewritten to the point that they are no longer canon within the context of the history of the Marvel Universe fans know best. These kinds of retcons are hardly the only way in which the bounds of what is considered canon is pushed. The X-Men's experiences with what could have been or what might have blurred the lines of what is and isn't real so many times that numerous events that shouldn't be canon absolutely are.
Source：https://www.cbr.com/xmen-best-alternate-universe-comics-deserve-marvel-canon/
