Officially go 1.21.0+ doesn’t support Win 7 anymore, that’s correct. However, in practice that only means “we’ve stopped testing with it”, which isn’t the same as “it won’t work”.
It’s go 1.21.5 (Syncthing crashing after updating to v1.27.1 / Windows 7 / runtime.asmstdcall() - #2 by calmh) that happened to include a code change switching to a CNG function that doesn’t exist in legacy builds of Microsoft’s CryptoAPI. That’s what caused an effective “does not work anymore”. Of course there may be even more incompatibilities that haven’t manifested into something noticeable before things broke.
TL;DR: The last syncthing version that supported Win 7 was 1.23.7. The last version that is known to work for Windows 7 is 1.27.0.