Here is my 2 cents... There are a few routes to go here, but it's mainly about tagging (except #3).
Option #1
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).
Option #2
We can try to tag all existing plugin dev questions with "craft-2". There are probably hundreds though, so this could take awhile.
Option #3
We make some sort of actual note (whether an edit or a comment) to all outdated questions.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.
Final thoughts
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!