And to answer the second half of your request, here is that exact same code as above, but now using the 1.27 generic methods feature (with a runnable example using tip at https://go.dev/play/p/1YK62tGetsm?v=gotip):

  // Map maps from a slice containing type In to a slice containing type Out.
  func (s MySlice[In]) Map[Out any](f func(In) Out) []Out {
      var out []Out
      for i := range s {
          out = append(out, f(s[i]))
      }
      return out
  }

In short, you could always have methods on a generic type since Go first introduced generics in Go 1.18, but with 1.27, the methods on the generic type can also introduce their own additional type parameters.

(Previously, you could achieve the same net effect with a top-level generic function, but then the code would not be grouped as nicely as hanging it off of the type, and arguably it now can have slightly better ergonomics in some cases. You can see more of the rationale from Robert Griesemer at https://github.com/golang/go/issues/77273.)