list=recentchanges has rcprop=tags. Similar parameter is missing for list=watchlist. It would be useful to get the edit tags when retrieving the watchlist via API.
Personally, I am actually only interested in the wlprop=tags, and just noticed that the filter is missing, too, without wanting to use it. So I don't have any preference in whether to leave this task open until somebody implements the filter, creating a new task for it, or just ignoring it.