I understand the motivation for having a different tag for sequels. However, I think Overwatch 2 is an exception, as it's a live service game that completely replaces Overwatch 1 (which cannot be played anymore). As a result, I think it would be appropriate to merge overwatch-2 into overwatch, especially considering that all new Overwatch questions are likely from Overwatch 2 players.
I believe the tags should stay separate. Despite Overwatch being unavailable in place of Overwatch 2, questions for the original game are still largely unique. Heroes, maps, balance, and more, are very different between games. Going back to all the old questions and updating them to make sense for the new game would be silly, in my opinion. Keep the Overwatch tag as is, maybe with a message that the tag is deprecated with the game being unavailable to play, and make sure that users know to use a new Overwatch-2 tag.
For those asking, here are some questions I consider unique to the original Overwatch:
Why haven't I got the Legendary and Epic skins that come with the Overwatch: Legendary Edition? (there are no more lootboxes)
In what order are lootboxes opened? (see above)
Does Orisa's "Halt!" activate when it hits someone? and What is the range on Orisa's Halt if it hits an object? (Orisa no longer has the Halt! ability)
Does Orisa's Supercharge ultimate stack with other damage buffs? (Orisa no longer has Supercharger as her Ultimate)
How do I achieve max damage with Doomfist's Seismic Slam? (Doomfist is now a Tank, and no longer has the Seismic Slam ability)
I can go on, but there's 13 pages of Overwatch questions. If someone wants to volunteer to be a janitor and clean up 13 pages of questions with numerous answers, go right ahead, but I do not see any reason to merge the tags. The game is called Overwatch 2. Let's keep the new tag, and leave the old one for legacy purposes. Why knows, with Activision-Blizzard being the money hungry saps they are now, it's only a matter of time until Overwatch Classic comes out, and the questions are relevant again.
The tags should be synonymized: overwatch → overwatch-2 but not merged.
(See the Meta SE post, What are tag synonyms and merged tags? How do they work? - for the difference between tag synonyms and merged tags.)
If we do it this way, then questions currently tagged overwatch and overwatch-2 will stay as they are, with no retagging (no merge). Since some overwatch questions may not be relevant anymore nor make sense for Overwatch 2, we should not merge the tags, and only synonymize them.
No. We should rename overwatch to overwatch-1 instead.
- overwatch-1 and overwatch-2 questions will stay separate, avoiding the disadvantages that come with merging or synonymizing these two tags, as already mentioned in the other answers and their comments.
- Prevents confusion, since some overwatch-1 questions may not be relevant nor make sense for Overwatch 2 anymore.
- No need for tag warnings, since the game's iteration is clearly indicated in the tag and tag wiki.
- Since overwatch-2 is not merged to overwatch, overwatch-2 will reflect the game's current iteration's official name.
This idea is from @V2Blast's comment at Tag Warnings: Looking for suggestions of tags that often got low quality questions and would benefit a warning when trying to add that tag :
Also, something else we've done over on Role-playing Games with the tag for the 1st edition of an RPG system with multiple editions is to rename the tag from, e.g., [adnd] to [adnd-1e]. This helps avoid folks mistakenly thinking it's a tag for any edition of the game, rather than a tag for a specific edition. In this case, the mods here could just rename the tag to [overwatch-1] instead.
(Adding another answer because this is a different suggestion to my initial answer. I don't think that I should delete the other answer because it already has a few upvotes.)