Very low priority suggestion, fwiw:
It might be handy to have a way to query the current matrix stack depth.  (or alternatively, to query amount of stack space remaining)  So you could tell in advance if pushMatrix() is going to fail.  Would be nice for recursive things where depth isn't known in advance - could be used to break out of recursion when limit reached.  

[edited] oops, forgot to say why:  point would be to avoid having to catch exceptions -- i don't mind try/catch myself, but seems "alien" to the p5 api[/edit]

Understood.  But that approach just helps make the point:  users wouldn't have to reinvent a stack pointer that already exists if an accessor method were available.

It's a trivial issue, and try/catch does work just fine -- the only reason I brought it up is because it seemed out-of-step with most of the api design philosophy that otherwise tends (or so it seems, at least apparently) to avoid exposing the user to exception handling.