> If we reintroduce it, why not put it back at its previous place, to provide ABI compatibility?

I agree. If compatibility matters here, then we should add new elements at the end.

> First of all, we longer care about ABI compatibility of PyTypeObject.

Oh? In that case, let's just remove the reserved/deprecated field :)

> Second, it is extremely unlikely ...

As a rule, "extremely unlikely" isn't unlikely enough for a product with as much reach as Python has. Even if something only happens to 0.1% of users, that's still thousands of people. You have to be prepared to justify hurting those people - potentially to their faces - to back up ideas like this. (And FWIW, yes, I'm happy to explain to people that their pre-generated Cython code broke because Cython had a bug that has since been fixed.)