Tags from notes which are in Junk are still available in tags lists (add/filter) even if this is the last instance of the tag.
Personally I would prefer that the tags remain existing when in junk, because junked cards are not actually deleted. Also if you have lots of junked cards, leaving the tags available means that you can filter and query your junk to look for cards to restore.
If you create a new card with a new tag, then junk and delete it the tag will be deleted as well (you might have to refresh because I think tags are cashed)
There is some inconsistency here: yes, you can filter a card, which is currently in Junk, but you can not find the same card by searching… So, I still think filtering Junk notes is a bug, not a feature… 
Junk is not Archive. You put the card in Archive when you want to hide it. And you put the card to Junk when you don’t need it anymore (but are not ready to delete in permanently).
@Connor said there will be some kind of Archive in the future SN build. Let’s see how the cards will behave when Archive arrives.
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Ah fair point, I see what you mean now!
(also I think you mean undocumented feature not bug )