Oops, after several months, I just noticed that there is a typographical error here, and it is too late to edit my previous post in this thread. Rather than PEP 665, it is actually PEP 661 – Sentinel Values, which is discussed in great detail in the thread, PEP 661: Sentinel Values.
EDIT (March 7, 2022):
Thanks, all, for an interesting discussion, and for the patience you have shown regarding some departure of attention from the original topic. 