I think a 30% keyboard is a feasible everyday keyboard to use.
(26 letters, Enter, FN, Backspace, Space, Shift, and everything else on FN layers.)
I've thought about this, but realistically, it doesn't make sense.
Punctuation is too common to be skipped, at least two of them are way too common.
OTOH, Enter and Backspace could be skipped—you can use Control sequences in software instead, provided the software isn't MS Windows.
That puts you to 28 symbols, spacebar and Ctrl-Alt-Shift-Fn modifiers. Realistically though, the modifiers should be mirrored to avoid weird hand contortions. Moreover, there should be more modifiers to get all expected symbols/functions: if you count only non-letter symbols present on US QWERTY's primary layer, that's already ~26 (digits + special punctuation).
However, with enough chording it's getting closer to a chording keyboard. Then you don't even need one key per letter.