What was it that broke it? Usage of unsafe? Security and bug fixes in the compiler/tooling? Just interested.

The Go 1 promise[0] says that the language spec and set of core libraries will be backwards compatible. I maintain a few dozen go projects, some of which are 10 years old, and while upgrading from i.e. 1.4 -> 1.18 has been relatively easy, it definitely has not been zero work. There are changes in the tooling, dependency ecosystem, deprecated dependencies that should be replaced, runtime behavior differences, things like that. But the language itself and stdlib is totally stable.

[0] https://go.dev/doc/go1compat

I didn't try for a long time I have a hard time remembering. I believe it had something to do about how it handles dependencies.