This advice of "returning concrete types" is in most cases a horrible anti-pattern that prevents evolution due to lack of information hiding. It has been also deliberately broken in the standard library in several places. This "advice" cannot be generically applied. Places where it is has been deliberately broken:
net.Dial (Conn, error)
image.Decode(r io.Reader) (Image, string, error)
sha256.NewXXX() hash.Hash
flate.NewReader(r io.Reader) io.ReadCloser
http.NewFileTransport(fs FileSystem) RoundTripper
Regarding `os.File`, the Go team even said: “If we were starting from scratch, we might do it differently.”
That’s why Go added abstractions later like fs.FS and fs.File.
embed/fs.Open again deliberately breaks this.
Whereas consider its counterpart net.Conn. net.Conn is one of the most successful interfaces in the Go standard library. It’s the foundation of the net, net/http, tls, and net/rpc packages, and has been stable since Go 1.0. It didn't need a replacement fs.Fs.
If you will always only ever have one implementation in absolute permanence and no mocking/fake/alternative implementation is ever required in eternity, return a concrete type. Otherwise, consider whether returning an interface makes more sense.
