`invalid-sentinel` - why does the argument have to be a literal?

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https://pyrefly.org/en/docs/error-kinds/#invalid-sentinel

# First argument passed to sentinel constructor isn't a string literal
my_str: str = "MISSING"
A = Sentinel(my_str)

this requirement doesn't seem to be mentioned in the docs or the PEP, but interestingly pyright also reports the same error. (ty doesn't)

i'm wondering if theres some documentation somewhere i'm missing, or if this just came from an early draft of the PEP that was later removed?