Another point maybe worth discussing in the PEP is “sentinels” vs “symbols”.
In languages that have symbols, they work like “duck enums”, in that 2 independent modules can construct the very same symbol.
It seems to me from reading the Rejected ideas there is concensus that’s an anti-goal. We want a way to get a “dedicated and distinct sentinel” without worrying someone else would produce it (other than by importing from the module where it was defined).
Plus Python has a long history of using regular strings (or interned strings when is matters) in most scenarios other languages would use symbols.