The Go team is addressing that in the release notes: https://go.dev/doc/go1.27 They think it will only affect use cases where a high number of idle connections were allowed to linger, for instance by setting MaxIdleConns in Transport to 0. They recommend to disable keep alives in that case.
> for instance by setting MaxIdleConns in Transport to 0
Which a lot of libraries are doing because they wrote an http.Transport{...} literal in an earlier version, and then std added new fields to the type in a way that silently breaks existing users. The zero value should have matched the previous default behavior.
We had the same in our own library, and now have a testcase checking if our own custom instantiation of http.Transport matches http.DefaultTransport, so that the tests scream loudly when upstream pulls this shit again: https://github.com/sapcc/go-bits/pull/309/changes
