We have gotten a liiiiittle more liberal ever since we introduced the new GODEBUG feature flag mechanism.

I've been meaning to write a "how to safely update Go" post for a while, because the GODEBUG mechanism is very powerful but not well-known and we could build a bit of tooling around it.

In short, you can upgrade your toolchain without changing the go.mod version, and these things will keep working like they did, and set a metric every time the behavior would have changed, but didn't. (Here's where we could build a bit of tooling to check that metric in prod/tests/CLIs more easily.) Then you can update the go.mod version, which updates the default set of GODEBUGs, and if anything breaks, try reverting GODEBUGs one by one.

That sounds good.

Breaking changes in major version updates is a completely normal thing in most software and we usually check for it. Ironically the only reason we weren't previously bothering in go is that the maintainers were historically so hyper-focused on absolute backwards compatibility that there were never any breaking changes!