My main concern is that types that were immutable in previous Python versions shouldn't become mutable as a surprise side effect of making them heap types. I don't know which types have become mutable this way though.

My other concern is that the language design *intentionally* disallowed mutating built-in types (unlike some other languages that allow it, e.g. Ruby). Mutating built-in types is one of those "attractive nuisance" anti-patterns where at a small scale this often appears to be the quickest way to solve a problem, but it tends to break unrelated things in larger-scale  applications (when different libraries using such tricks collide).

(There's also a concern about mutating types that are shared between multiple interpreters, but IIUC heap types are not shared, so this shouldn't be a problem.)

Presumably we should review the list of heap types that you are proposing to make immutable (in a form more easily digestible by humans than a diff) and reach agreement on that. And perhaps the list should include information on when a type became a heap type. (Types that were always heap types probably needn't be changed.)