Standard library APIs like context.Context's WithValue/Value, sync's Map/Value/Pool, stuff like the sort or container/heap packages. Some of them are obvious "really wanted generics, but wasn't important enough to get the special built-in treatment" cases, some aren't.

Most logging libraries I've seen follow the pattern from fmt.Println etc to accept arbitrary `interface{}` values to be logged instead of actually using some Stringer or Formatter interface in their actual method signatures.

Context's WithValue will be an interface even if go supported generics. It stores anything, not values of a specified type. Same with fmt

Yeah, that's what I meant with "some aren't", I was mostly listing stuff to support the idea that `interface{}` itself is ubiquitous and not some super-niche edge case that never shows up in real code.