It is silent until you press ^t, and then it prints the current progress. It's not constant output.

Sure, but the BSD convention meets that until you hit ^T, and then it does the thing the user wants (give a report) and then it goes back to being silent. There's nothing wrong with that, and the BSDs are Unixen, so... It's not like ^Z (SIGTSTOP) is un-Unix-like just because Unix VII (and System V) didn't have it!

Unix-like sounds unreasonable if there isn’t leeway for optionally reporting progress on stderr (--quiet/--non-quiet or similar).

No news is good news

UNIX-like is having your session or app time out because it gets no output. going all the way back to HP Omniback 2 running BCV split pre-exec scripts on an EMC array to back up a database to tape off a split mirror. Then aborting the backup because the array-side mirror split took over 15min, with no output.

UNIX-like is not a convention of UNIX - it's a convention of "we didn't get to making that useful thing yet, there's other stuff to do."