>>100978775In Superman's case, it's mostly completely non-canon due to the Byrne reboot bringing some core changes to various characters.
In-universe, the justification is that history itself was changed when the Earths fused at the end of Crisis on Infinite Earths, but the effect was delayed, which is why that story ends with Pre-Crisis Superman still around, seemingly with no changes.
For other characters, it varies. The solo stories of the JSA members from even the 40s were still considered canon post-Crisis (aside from the Golden Age versions of Batman, Superman and Wonder Woman), but the actual JSA adventures often were supposed to be "tweaked" in modern continuity, even if they still happened in rough lines, like removing or replacing Wonder Woman (Batman and Superman weren't an issue since they only made one appearance, before the JSA's later appearances in the 70s).
