Anyone with an older toolchain is free to fork it on github, test with the older version, and CI to the project that tests with the older version, and submit a patch, too!
This may not get the project as many users, but not everyone who writes a 50 line project is trying to figure out which versions it supports and setting up full test matrices either.
Not a Go dev, but I typically set up a CI with the oldest toolchains I support (usually a debian release), and only bump those versions when I really need something from the latest versions. Locally I build with the most recent tools. This ensures good enough coverage for very little work, as I notice when I start using something that's newer and can bump the toolchain accordingly.
Sure, but if you start a new small project and throw it on GitHub, it's not totally insane to just put the version you tested. Just because someone put up their tiny library doesn't mean they've put in the effort to figure out which version they need.
This may not get the project as many users, but not everyone who writes a 50 line project is trying to figure out which versions it supports and setting up full test matrices either.