As a semi-professional trombonist, C clefs are very important to me. Alto and tenor as-written, tenor as a means of transposing Bb treble, mezzo-soprano as a means of transposing F parts… they are not obsolete.

Yes, I did write "mostly". Sometimes the bassoon uses the C clef as well. Still, overall it can be safely ignored in most cases.

My favorite genre of hacker news comment is when someone lacks sufficient imagination to wonder why something that’s commonly used by many people is still in use

FWIW, there's no real need for imagination here. The comment says "mostly", and added a parenthetical comment that was simply meant to name the viola as a prominent remaining use.

In particular, trombones have almost entirely stopped using the tenor clef in jazz.