Sure, it's useless in plus-only servers, but it's also the only way to tell if someone does not run plus and then teach them how to install it. Most recent new players did not have plus.

I'm not against keeping it in case /plusonly is off. Maybe make a poll for it?

6.6 draws a red - sign next to players who aren't running plus, which is much more rare and worthy of attention, rather than draw a + next to basically every player, which is not helpful. But it looks like it only draws that - for the server right now, not for clients, so that could be changed easily... it looks like clients do probably do get sent correct information about whether clients are running plus or not.