They should perhaps be added. Ideally not just for the public API surface, but for the whole code base. This could then replace quite a few Debug.Assertions that guard against null references internally.

(This might potentially be a good first issue for new contributors. I expect that there will be some code locations where it isn't immediately clear whether null is allowed or not, and where one would have to study the code & execution paths through it for a bit... but even that shouldn't be too tough.)