We certainly don't want to use question tags to mark questions as no longer relevant, and honestly I think that the only correct way to mark this content as such is with the soft pink shade of deletion.
The whole idea behind organising information on Stack Exchange is that you would have a question, and you would Google for it, and we would have a laser-focused answer, and the answer would be always correct because it's really easy to edit existing answers or write new ones altogether.
Okay, but now the mechanic is removed. The game is updated for everybody, or almost everybody. The world moved on. So should we.
Ask yourself: as a new player of this game would I ask this question? Would I change how it's phrased? Can I do that while keeping the current answers relevant?
If the answer is no, why would we keep the question? What good does that do?
We really can't hope to get away with a little disclaimer tacked on somebody's answer. Best case scenario the disclaimer reflects the current state of the world ("followers no longer gain XP at all") and as a result the rest of the answer is useless and wrong. Worst case scenario, we are copy pasting the same blurb on a bunch of questions ("This no longer applies to the current game, sorry") and that it's just a waste of time for everybody visiting the page.
We may get some kind of feel-good warm fuzzy feeling in our hearts knowing that we are providing archaeologists the means to discover what the world of trading in Team Fortress 2 was before Steam Trading happened. Archaeologists however are not our audience. Our audience is by over 90% made of people who are making a Google search because they need help here and now.
We should focus on them, and we would do them a disservice to keep wrong, irrelevant trivia about the past that's never coming back.
We have been very aggressive in making sure that we are no host for questions that can't be given relevant answers, and we are being richly rewarded for it. I wish that we would be just as aggressive in keeping our content fresh and relevant despite how much harder it is with potentially hundreds of questions on a single tag rotting all at once.