Luckily the pronounciation of finnish languge gives MUCH less use for de-essers than many other languages. I've never needed one for vocals. This HoRNet-collection seems to be worth investigating, though.
Listening to, say, polish female singers I think de-essers must have originally been invented in Poland :o)
Female british, (especially) Oxford accent can be a nightmare, too. I remember, about 25+ years ago, the "s" of a british
actress was so whizzling, that it caused blackouts in the TV-broadcast several times during the movie because it was close enough to the signal used to automatically shut down the station when broadcasting was finished for the day.