This is a dangerous pattern for your community to fall into. Here's why:
Version tags are redundant. The site is already called "drupal". We don't need to know that it's drupal-6, drupal-7, drupal-8, drupal-9 in every single tag on every single question. It's redundant.
Version tags are a crutch. Because these tags dominate the site, and contain the name, they are the first thing users will turn to when tagging their questions. This means users will fail to tag questions with any other meaningful tags, since "I already tagged my question with drupal-6!" This isn't hypothetical. We've seen this happen time and time again. Version tags are a mindless replacement for thinking about what your question is about. "Oh, this question is about drupal-7. Done."
Version tags make questions disposable. The entire point of Stack Exchange is for questions to be editable, timeless resources -- a version tag implies exactly the opposite and ties questions to specific moments in time, with no incentive to improve them to be relevant to future versions. This is extremely dangerous!
Version tags encourage needless question duplication. Rather than "How do I do X?" which can cover both supported versions, now there has to be "How do I do X in Version 1?" and "How do I do X in Version 2?"
Version tags aren't necessary for most new questions. New questions are very likely to be about the current version of the software -- so for all new questions, tagging with the current version is usually not helpful.
On top of that, how many versions (outside of freakish "I still use Windows 2000" edge conditions) will really matter at any given time? Two at best? And certainly any OSS project worth its salt will strongly urge people to regularly migrate to the latest version as soon as they can, so that the community can move forward to bigger and better things.
Thus, here is what I propose.
Eliminating the version tags on existing questions. We can do this in one click globally.
Treating version tags as the exception not the rule on new questions -- if you have a strong case to make that the question you are asking is ONLY relevant to one version and can NEVER EVER be relevant to any other version, then -- and only then -- add the version tag.
If you encounter an old question that is truly specific to a version, and cannot be edited to be version-agnostic, retag it with a version tag.
The bottom line is that version tags have to be treated as the exception, not the rule. It is important to fix this now while your community is young? Otherwise you are creating some deep problems in your community that will be very hard to fix 1 or 2 or 3 years from now.