The link register isn't reserved in either AAPCS64 or Apple's ABI so it's effectively 29 GPRs. Yes it has a specially-defined purpose at function bounds, but it

The link register isn't reserved in either AAPCS64 or Apple's ABI so it's effectively 29 GPRs. Yes it has a specially-defined purpose at function bounds, but it's perfectly fine to use it for whatever within a function, just like it was in armv7.