Actually Unix does use JSR with a non-PC register to hold the return address very extensively; but it also uses the 'saved PC points to the argument' technique; although only in a limited way. (Well, there may have been some user-mode commands that were not in C that used it, I don't know about that.) First, the 'PC points to arguments': the device interrrupts use that. All device interrupt vectors point to code that looks like: jsr r0, _call _iservice where iservice() is the interrupt service routine. call: is a common assembler-language routine that calls iservice(); the return from there goes to later down in call:, which does the return from interrupt.

Use of a non-PC return address register is used in every C routine; to save space, there is only one copy of the linkage code that sets up the stack frame; PDP-11 C, by convention, uses R5 for the frame pointer. So that common code (csv) is called with a: jsr r5, csv which saves the old FP on the stack; CSV does the rest of the work, and jumps back to the calling routine, at the address in R5 when csv: is entered. (There's a similar routine, cret:, to discard the frame, but it's 'called' with a plain jmp.)