See original discussion on python-dev:
https://mail.python.org/pipermail/python-dev/2017-December/151328.html

Summary: binary compatibility for C extensions which don't use the stable ABI is currently "best effort" and uses distinguished flags in the tp_flags field of type objects to indicate whether a field is physically present or not.  Unfortunately, tp_flags is 32 bits and therefore a scarce resource.  Also, binary compatibility is 1) not guaranteed anyway 2) of less use nowadays.

So we propose to remove the binary compatibility requirement when creating static type objects (i.e. not using the stable ABI).