So after upgrading to 1.12.1, the Update Modules tab tells me
Updates of Backdrop CMS core using the web interface are disabled.
I had to go googling and then searching the code to figure out why I was getting that - turns out I have to manually update the installer.settings.json file. Shouldn't that be in the release notes?
Sorry, this was user error.
For any future users looking how to turn on manual (or automatic -- in the future) core updates via the UI, look in admin/reports/updates/settings (Reports > Available Updates > Settings) at the bottom of the page.
Today, once again, I had forgotten where this setting is to be found and I cannot find it mentioned in our user guide or developer notes. Is it there somewhere? Shouldn't it be? Thank you for posting it here!
Sorry, this was user error.