https://docs.python.org/dev/c-api/typeobj.html#Py_TPFLAGS_IMMUTABLETYPE says:
"This bit is set for type objects that are immutable: type attributes cannot be set nor deleted."

Is it possible that a metatype (type subtype) overrides tp_setattro and ignores the Py_TPFLAGS_IMMUTABLETYPE flag? Should we suggest that metatypes implemented in C and overridding tp_setattro should take the Py_TPFLAGS_IMMUTABLETYPE flag in account?

When a type overrides tp_setattro, I understand that it's fine since it's only used to set attributes of its instances, not on the type itself.

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See also bpo-43770 "Rework C types initialization": some types explicitly sets explicitly tp_setattro to PyObject_GenericSetAttr. But it's unrelated since it's used to set attributes of type instances.