Thanks, @Simon, for the detailed path. I must have gotten there in time, because it said there were no backups (and backups were using 0 KB).
Not sure if this is the right place to report this, but my old iPhone just got a security update to 15.8.1 so it is still in the loop for updates.
It’s surprising that it took so long to appear since 15.8.1 was from January 22.