What perplexes me is that when I tried logging in on a couple of other sites, and deliberately provided an invalid password, I got an error message from my ssh client saying "Password invalid, please try again": when logging in on sh.sonic.net, I just got the password prompt again with no error message. I turned on verbose logging (-vvv) in the ssh client, and it didn't show any error message coming back from the server: it just re-prompted for the password. To me this suggests strongly that there is a problem with the server: the OpenBSD client, at least, is pretty recent (2018), so it seems unlikely that there's a protocol incompatibility.
I hope someone can help me past this problem. I don't want to do anything exotic with the shell account, mostly just use it to check the behavior of some common Unix utilities when my desktop version doesn't do what I expect.