To quote directly from the
policy:
So:
- 5e has its own distinct, separate canon from the other editions, which definitely includes the current core, but they don't confirm what is canon for 5e beyond that. Though odds are that in practice, new 5e stuff does replace old 5e stuff, as you say. (We do know a few things that aren't canon in 5e anymore, because they were marked as Legacy Content on D&D Beyond.)
- Every other edition also has its own distinct canon. They clarify this with an example of how 4e differed from the canon of previous editions (and by implication 5e, since succubi also aren't devils in 5e).
- Other expressions of D&D beyond the RPG (novels, video games, movies, comics, and presumably other media) are also treated separately from each edition's canon.
- Each canon may or may not include lore from other canon (the "not necessarily canonical" bit). But they are still treated separately from one another by default, and 5e very specifically does not "adapt the canon of earlier editions" wholesale.
You are correct, we don't know what the policy was in 4e or earlier. Though 4e certainly seemed to be a different continuity from the previous editions - something the 2021 policy even calls out - that doesn't seem to be the case with 1e, 2e, and 3e. Though as noted earlier, even between those editions there were retcons, some apparently controversial with fans (such as in Tales of the Lance).
As for 5e, I would agree that the 2021 policy seemed to be a recent change: Ravenloft earlier that year was the first 5e setting product that radically broke from earlier canon. They certainly seemed to have a different idea in 2014, since the original 5e DMG notes:
Other references to older settings in the 2014 5e DMG similarly point inclusively to all the media that came before.
This is true. Notably, the policy disappeared from public view when they moved the old D&D website over to D&D Beyond. Perhaps that could indicate they've reconsidered.
However... they haven't provided a new policy. And setting updates since 2021, up through the 2024 DMG, continue to reflect the approach in that policy (albeit less dramatically than Ravenloft). So I see no reason to think they've changed their mind.
Now, folks are certainly welcome to dislike the policy and/or ignore it for their own unofficial efforts (such as the FR wiki). I'm not asking anyone else to embrace it. But I am taking Wizards at their word, that this is how they treat the official material now. And I think it's an interesting thought experiment to see what that looks like. Especially for settings that didn't get a dedicated treatment in certain editions, like Dragonlance.