Dropping support for a group of platforms is definitely something I'd consider a breaking change.
I think that's a breaking change by most definitions.
This is particularly nasty when it happens in mostly stable, rarely-updated internal projects because what was supposed to be a 5-minute, 3-line update turns into a several hours long wrangling match. Apparently, the code does rot and rust or something.
https://github.com/grpc/grpc-go/issues/3798#issuecomment-670...
See the discussion on semver and experiments: https://github.com/semver/semver/issues/238
If you want this project to use a particular versioning scheme then you should just argue for that directly.
[1] Other than this snippet :) : “on curl’s 25th birthday made it extra fun”