This forum thread pitched making @convention(thin) an official feature, but I'm not finding anywhere that it was proposed on Swift Evolution or approved. It appears only once in the SE repository, in SE-0426, in a list of types of functions that can conform to BitwiseCopyable.
The Swift book has reference for @convention and notes in a hidden comment that @convention(thin) is private, even though it isn't underscored, with a link back to a forum thread from 2018.