I obviously don’t think that is wise, but Git is literally designed with this in mind: https://git-scm.com/docs/repository-version/2.39.0

Just like SQLite has an explicit compatibility guarantee through 2050. You literally do not have to update if you do not want to.

And it’s still a choice you can make regardless of Git moving to Rust or not, so what’s the problem?

This is the repo format version.

It's pretty different from the git version, which receives new releases all the time for things like security patches, improvements, and new features.