UPD: Answer: If wanted, should be placed in an annotation of release level.
Is it “miscellaneous roles / miscellaneous support”, task “special thanks” or in an annotation? If in an annotation, then should it be placed on recording level, release level or work level?
I would say - annotation.
Many of those thanked will be difficult to identify and are most often not involved in the production.
Personally, I don’t enter thanks credits at all.
Usually, thanks are given at the release level. 
Agreed. Don’t add them at all, but if you want to, should only be in annotation. But it’s the one of the credits in booklets that I don’t think really have a place on Musicbrainz at all.
OK, thanks both to you! Then as with patrons I will place this info in an annotation to not litter DB with artists, that are not linked to musical productin. Like there, my opinion it is part of musical knowledge and artist/release/recording history, even if it is not a direct participation in musical production so it is still has a right to be mentioned/saved in history and DB.