sys module documentation (as it is online) has some things that in my opinion should be marked as implementation details, but are not. Feel free to counter why not. 

Some of them has info it should be used for specialized purposes only, but IMO it's not the same as not mandatory for other implementations.

Temporary list:

_clear_type_cache

dllhandle

getrefcount

getdlopenflags (?)

getsizeof - it might be not well defined on other implementations

setdlopenflags

api_version