Originally Posted by
Prof. Warren
Both. The sliding timeline is a tricky thing, one that requires you bend your mind around certain discrepancies.
The original stories still count and are still canon. They happened as depicted, whether they're from the '60s, '70s, and so on.
Of course, because in-universe, the beginning of the MU is always sliding further up the timeline in order that these characters can still be the age they're supposed to be, you have a situation where characters like Peter Parker and the X-Men had their early adventures in the '60s and yet if you reference those early adventures in a new story today, writers have to either ignore the anachronisms or insert references to modern tech or pop culture that would apply to, say, the '90s or early 00's rather than the '60s.
This doesn't mean there's been a wholesale rewriting of Marvel history, it's just how the sliding timeline is addressed.
Tony Stark originally became Iron Man in Vietnam, for instance, but that isn't the case anymore. They haven't explained how that isn't the case anymore, but when his origin is referenced, it's just a more recent war that he was wounded in. It's just assumed that fans understand why things are updated and will just roll with it without needing an in-universe explanation as for why the original stories are about Tony in Vietnam but now it's, say, the Gulf War.