Anyone who has made a serious study of classical guitar technique is likely to have become adept enough at barre-chording not to use that good old "hooking the thumb over" left-hand technique.

You get the strongest left-finger pressure and the longest reach by keeping your thumb positioned pretty much at the center of the neck. This is not particularly intuitive, but if you study and practice and drill as a classical guitar student/player, this left-hand positioning should become second nature, and you'd never want to grab the neck with your fist is such a way that your thumb would ever get anywhere near a place where it could fret a sring.

There are a few riffs in a few pieces in the folk/blues/ragtime repertoire that require the use of the left thumb on the fingerboard if you're trying to play exactly like the originator, e.g., Rev. Gary Davis, Blind Blake, etc. But of course, not everyone desires to play those songs, or at least not in precisely that style. A skilled practitioner of classical technique can play a whole lot of very tricky stuff, and could probably come up with an alternative way to play most of Blake's and Davis's most difficult passages, or at least closely approximate them.