On mobile frontend, you are given the option to add the watchlist, as well as other special page, to your watchlist (only visually)
Steps to Reproduce:
Go onto the mobile version of any Wikimedia project.
Open the search menu.
Type "Special:Watchlist" (or any special page).
See the watchlist star appear next to the result.
Indicates that adding the page to the watchlist was a success, even if the action is not possible
You can't add it you your watchlist and the option is hidden.