The calling code can either bind a specific number of return values (implicitly 1 unless you manually bind more), or capture all returned values into a list with no expectation of count. The latter is usually used in interactive & debugging tools more than anything else, so the former is the common case.
However, it is never an error to have a mismatch when expecting a specific number. If there are more than expected, then the rest are ignored; if there are fewer than expected, the remaining bindings are set to NIL. Neither case causes an error. So technically, the caller always “cares” but doesn’t know, and freely accepts whatever it gets at runtime.
It’s hard to say how often a mismatch occurs in calling user-written & library functions, but it’s relatively common in calling the standard functions: Hashtable accesses, mathematical floor/ceiling/truncate/round, file reading functions, parsing strings, and many others return multiple values which are commonly used only for their first return value. I don’t have the infrastructure to directly measure percentages.
I do think it counts as “genuinely dynamic”, as you describe above, at least in the compiler assumptions for current platforms. What is the hit on an 8-slot belt for having 25% of the belt taken by every return, instead of 12.5%? Do values swapped to the scratchpad or spiller for short term use effectively cost little to nothing?
The Mill has some “global” registers like the thread-local pointer. Are there some that are available for user code as well? They’d be handy here to extend the calling convention, or just in general to extend the ABI that dynamic & GC’d programming languages can build their infrastructure from (slab allocation pointers, multiple stacks, current closure, dynamic environments, local/scoped constant tables, etc). On SBCL PowerPC, a whopping ~14 registers are globally reserved for such infrastructure (though some just hold constant addresses for speed & compactness). Smaller register architectures have to trade off which of these very commonly used pointers will be offloaded to RAM.