This feature seems to be driven by Instagram use case. Are there other users who would benefit of that? Is it a common use case to load big data and then fork to use preloaded data?

PR 19474 has a maintenance cost:

* new configuration option
* new macro
* need a buildbot to check that the option is not broken
* document the change
* etc.

There is even now a question about using a different ABI flag.

It's not the common case to build a custom Python manually for a specific use case. Most users use a binary shipped by their operating system.

I'm not sure that it's worth it for Python to maintain such special build.

Maybe bpo-39511 would be a better motivation to support immortable objects.

But I'm also concerned by the huge impact on performance :-(