Your Trakt lists, from any poster
Long-press a poster and choose Trakt lists… to file it into one of your own
lists, or take it out. Your watchlist is not in there — that is what the library
row already does.
Which lists already hold the title fills in a moment after the dialog opens.
Trakt has no way to ask "which of my lists contain this", so it is one question
per list, and waiting on all of them before showing anything would be worse.
Removing from Simkl asks first
Simkl stores a title's list state and its watched history as one record. Removing
it from your library there also clears every episode you had marked watched — so
it now asks before doing it.
Removing from Trakt does not ask, because on Trakt it only touches the watchlist.
Nothing else is lost, and a warning that fires when nothing is at stake is a
warning people learn to dismiss.