As is, ISO compliance of these things is a minor issue in portability. The main issue is that the ISO core is too small and any serious program thus needs facilities that are outside the ISO core. There is still a bit of fairly common ground, but quickly we need features that are often around in most large implementations, but with serious differences in syntax and semantics. Next we have non-functional features that heavily affect porting serious applications such as differences in clause indexing, (atom) garbage collection, limits (arity, atom length, Unicode support, unbounded integers, …)
All these things are simply more important than raising an error on floor(1) and similar details. A large number of extensions are relied upon in the SWI-Prolog libraries, so a strict ISO mode should either be module specific or a lot of work needs to be done to make the libraries run in a strict ISO mode.
If you want strict ISO, use GNU Prolog or some other Prolog system that strives for that. With a few tweaks the result will run in SWI-Prolog. Porting in the other direction is typically a dead end for a serious application.