From Special:EditWatchlist, I select about 1000 items using the Shift+Click trick, then I press Remove button.
No pages are removed: the page is only reloaded and some last items I had checked are unchecked.
No error message is displayed, POST request returns an HTTP 200 state code.
If I press Remove button again, the same happens.
I tried to uncheck about 20 items in addition to those which were automatically unchecked, then submit. In this case, an error message is displayed (something like “not all pages can be listed”) but all selected pages seem to be well removed.