If it's a feature needed for the standard library, then it's safe to say the devs will need it to. The devs aren't (shouldn't) be consistently writing programs that are more trivial than the standard library.
I could easily write a custom Append func in my own application because I would know the Slice types I'm trying to operate on. Writing a language feature is different than writing an application, and it has nothing to do with how trivial something is.
Honestly, Append() as a concept is about as trivial as it gets..
More precisely, you would write one custom Append function per Slice type. AppendString, AppendInt, AppendWhatever.
True enough. But libraries can easily tend toward either of those extremes. And any obstacles to library development will eventually become obstacles to application development.