5E has
an official stance on canon, and it's definitely not "pick and choose." It's "every edition of the roleplaying game has its own canon" and "beyond [the] core rulebooks, we don’t have a public-facing account of what is canonical." 5E canon does not include older-edition canon unless they specifically include it in 5E material, and even then it's only a maybe if it's not in a core rulebook. (This latter point was recently underscored by them moving VGTM and MTOF into a non-canon "
Legacy Content" status, replaced by MOTM.)
Now, you can certainly pick and choose anyway - I do - but 5E isn't providing explicit support for this. It's something you just have to do on your own. Same as if you wanted to run a Greyhawk game under 4E rules, or an Eberron game using BECMI.
The funny thing is, tapping into the "vast back catalogue" seemed very much to be the plan for 5E at the beginning, which had references galore to specific older settings in the core rules (mainly in the DMG). They even included novels and video games in the lore. This continued into early adventure modules, with their guidance for adapting them to out-of-print settings. But something changed by 2021, as the new policy discourages that in favor of only 5E's material being canon for 5E.
I assume this is a reference to Klorr? That's more of an Easter egg, a sop to veteran fans, than encouragement to use older material. And even then you'd be expected to use it within Klorr, in the context of them all being "lost and failed domains" slowly sinking into the void. The "real" Ravenloft for 5E is the versions of the domains outside Klorr; the old versions are intended to be out of the picture.
Tons of fun.
