Thanks for sharing, quite interesting technique.
How do you use those offsets, is that for memcmp style comparisons?
Re the question itself about those SPIs. My humble opinion:
- de jure – they could change
- de facto – it is extremely unlikely they will.
- The other (IMHO worth taking) risk is for Apple starting rejecting app store apps that use those private symbols.
I'd take the approach of only starting worrying about those things when (and if) they actually break, especially when you have a workaround strategy in mind (plan B your hacky implementation, plan C the original slow implementation, or we may have something better in standard library by then).