We'll need a new tag
I think from what I have heard about the new PHB, it is obvious that we will need a new tag, because many of the answers that are based on the D&D 5e 2014 rules are just going to be plainly wrong once the new rules come out, and it is assured that the new rules will supersede and replace the old rules — that is how WotC has done it for all their other new publications, like Mordenkainen Presents: Monsters of the Multiverse vs the older Mordenkainen's Tome of Foes. And it will sell more books.
Just one example, in the new rules, wild shape does not assume the hit points of the new form any more like it did, it now retains the caster's hit points, and gives them temporary hits on top. This invalidates a lot of the old Q&As about wild shape. Or Rangers and Paladins now have spells from level one, and their features have been massively rewritten, so much so that Crawford claims the ranger is essentially a new class. Or, all classes with fighting styles now get access to all the fighting styles, and there are more fighting styles than in the old core rules. All classes that focus on weapons now get new weapon mastery features. There are so many old answers that are based on the dnd-5e rules that will be plain wrong once you play with the new rules.
Whether they call it that or not, this is essentially a new edition of the game, one that is just backward compatible enough that you supposedly still can use old splat books and adventure modules with it. It is like dnd-3e vs dnd-3.5. I also think that like with those two, there will be very, very few questions that are about both editions, and if you have them, you can just use both the old and the new tag. I'm not aware of a single tag that refers to all the various flavors of 3e, and think if it is not needed there, it will not be needed here.
Thomas' old answer is "let's cross this bridge when we get there", and that still applies, but we are nearly there. I almost would have posted a question already, because a lot has been disclosed in promotion videos, but I refrained just because the exact rules wordings are not yet public, and also because there was no tag for it, and I did not want to just bash ahead and create a dnd-2024 one. In that sense it is good to discourage premature questions, but it still might make sense to have some discussion about how to tag this new version of the rules.
The way the designers talk about it is they call it D&D 5e 2024 vs D&D 5e 2014. I'm not sure how to handle this new tag need, and there are community members that have a lot more experience with it than me.
My naive approach would be to just create a new tag called dnd-2024 or dnd-5e-2024, for questions that are about the new rules set specifically, which we might get a lot of, once the established and well understood rules set has been replaced, and continue using the existing dnd-5e one for the existing rule set, so we do not need to retag over 25,500 questions.
Maybe retagging can be done programatically and would not be an issue at all, so that we could retag all the old ones with dnd-5e-2014, and then use dnd-5e for the current edition. Especially for people who have no clue there even are other rulesets than D&D, or that there are other version of D&D than the one they just bought, having dnd-5e be the current version may save us a lot of cleanup work and typing.
Or, for some transition period of a year or so, we only supply the year-based tags, renaming all current dnd-5e to dnd-5e-2014 and offer that and dnd-5e-2024, then migrate that one to dnd-5e again later on, or make that an alias for it, when most of the incoming questions are about that version.
I agree with @Kirt in the question, and would be happy to have someone with more experience and battle scars of this make an experience-based suggestion how to best handle this, instead.
