A page in the Docs with the last known working version of Syncthing under a specific OS wouldn’t hurt, would it? Maybe not with download links and such, but just for informational purposes?
There are genuine cases where people simply cannot upgrade (e.g. old Macs) or are restricted by their environment (e.g. work, school, etc.). I can speak from experience regarding the education field - money is always lacking and combinations of ancient hardware paired with outdated software are commonplace.
Older topic with a similar discussion for reference: https://forum.syncthing.net/t/oldest-supported-macos-version/11611/6.