Bingo!According to WotC, 3.5 was "fully compatible" with 3.0.
And it was, for roughly a month after it came out. But the further into 3.5 we got, the less the new stuff resembled the original stuff. You could still use it, but then, I can still use my old 2e stuff too. So by that logic all editions and versions are "fully compatible" with one another.
After all, I have mostly used 1e, 2e and 3e adventures in 5e. I use 5e version of monsters and equipment but otherwise the story, the characters, the locations, they all work... I discovered that ALL editions are compatible!
For me compatibility can be summarized as: does this new rulebook work TOGETHER with the entirety of the rulebooks I already have?
If using a new rulebook means that I will stop using some of my previous rulebooks even partially, then they are NOT compatible.
And that only applies to rules including character material. It doesn't apply to fiction and props.