Separate to the ability to add labels when watching a page (T416291: Update action=watch API to accept label IDs), an endpoint should exist to add/remove labels for a given page(s) without modifying watch status, as part of T416615: Make watchlist labels usable via API calls and to support things like T416944: Add a page action to manage labels on a given page
This will probably still be under action=watch, but we need to make sure adding/removing labels using this doesn't affect the watchlist expiry value