It doesn't matter how old they are, just how long they were supported as main calls. So that seems to be up to FreeBSD10 if FreeBSD 11 introduced the new calls.
And since FreeBSD10 is not that long out support, if you support one set of system calls only, it is not that far off.
And if it is really that interesting to move from an AIXy calling convention that aligns parameters even in registers to save the zeroing of one register.
And of course the biannual rearrangement of the stat record with probably to make sure compatibility is certainly broken.
People that like that philosophy, probably also like submitting patches. I'm anxiously waiting.