Valve pulled support for Steam at the start of January 2024 for Windows 7/8. I thought that was the end, but apparently it actually just meant "Steam may still run but we don't support it in any way". Which surprised me when I booted up the old Windows 7 PC a few months ago and discovered that Steam still ran and seemed to work.
Apparently this update is actually incompatible and now Steam won't run at all.