Here is my 2 cents... There are a few routes to go here, but it's mainly about tagging (except #3).
We can tag all Craft 3 questions as "craft-3" moving forward. This is the easiest solution in the short term, but may cause more problems in the long run (see below).
We can try to tag all existing plugin dev questions with "craft-2". There are probably hundreds though, so this could take awhile.
We make some sort of actual note (whether an edit or a comment) to all outdated questions. This would be more obvious than a simple tag. But again, this would need to be applied to hundreds of questions.
Personally, I don't like Option #1. It seems like it's creating a large mess that we'd have to deal with at some point in the future. For example, if Craft 4 is completely compatible with Craft 3 (as Craft 1 & 2 were), then that tag would actually cause more confusion than it resolves.
Conversely, Options #2 & #3 seem like an awful lot of work. So really, none of these options seem like a slam dunk.
These are just a few suggestions... I'd love to hear what others think!