I think there's other cases of performance related features being hidden under an ifdef.  Computed gotos show up that way, although probably more because it's a compiler extension that's not supported everywhere.  Pymalloc is also very similar in that it implies an ABI change as well.

I wonder if it would be worth it to introduce an ABI flag for this as well?  On the one hand is it a slightly different contract, on the other hand using extensions that don't support the immortalization actually work just fine.