from __future__ import annotations

Mandatory as in coming by default. Optional as in needed to be "activated" from an from __future__ import annotations statement

Mandatory is an interesting word choice. I guess it means that it's by default in the language. You don't have to enable it with from __future__ import annotations

The annotations feature are referring to the PEP 563: Postponed evaluation of annotations. It's an enhancement to the existing annotations feature which was initially introduced in python 3.0 and redefined as type hints in python 3.5, that's why your code works under python 3.8.

Here's what optional from __future__ import annotations changes in python 3.7+:

class A:
    def f(self) -> A: # NameError: name 'A' is not defined
        pass

but this works

from __future__ import annotations

class A:
    def f(self) -> A:
        pass

See this chapter in python 3.7 what's new about postponed annotations:

Since this change breaks compatibility, the new behavior needs to be enabled on a per-module basis in Python 3.7 using a __future__ import:

from __future__ import annotations

It will become the default in Python 3.10*.

* it was announced to be default in 3.10 (when python3.7 was released), but it was now moved to a later release