But then what about the never-{attribute} option which is !async and !const.

That matches with !, logical not, it makes sense and reads naturally.

In comparison, in English, a leading ? does not. Rust could very well change this for Rust users in the same way programmers in general are familar with leading !, eventually.

Or just have it be const? and look nicer already. !async to mean "not async" just makes sense, its well established that !something means not something.

Also some attributes just make more sense prefix or postfix, Bouba/kiki effect, and this can be a valid reason for difference rather that "consistency".