Not quite true, we also have Ellipsis, which already has a nice repr that both reads easily and still follows the convention of eval(repr(x)) == x. It also is already safe from instantiation, survives pickle round-trip and is multi-thread safe.

So long as you are not dealing with scientific projects, it seems a quick (if dirty) solution to having a sentinel that is not None. There is also some symmetrical wholeness when considered with the other builtin sentinels: bool(None) is False; bool(Ellipsis) is True.