Backwards-compatibly changing existing features feels qualitatively the same to me as adding new features. In either case you can ignore them until you need to work on a codebase that uses them.
let found = match map.get_mut(..) {
..
}
if !found {
..
}
match map.entry(word) {
Occupied(mut view) => { *view.get_mut() += 1; }
Vacant(view) => { view.insert(1); }
}