If they are suppressing this

Yup. Must be. No “-U” found in man apt.

(Is there in man apt-get though. And it works on both.).

And it does not show up in fish’s option completion options on either, either.

Near 20 years of having been using apt-get (and later, apt), if I ever knew this, I forgot. Could have been doing just one command all this time.

what else aren’t they telling us

vim has a built-in autocomplete you can use by pressing ctrl-n during interactive mode.