An overhaul of that code is long overdue, and removing it to then put back what is actually needed (targeting 4.0+ only) might not be the worst way to do it. Apart from the signal_handler stuff (which needs to be implemented separately for each platform not providing fork) I think it's mostly small tweaks which may or may not be needed anymore.
Actually the most work I spent on getting Pike to run on AmigaOS was probably fixing breakage with a statically built Pike (as dynamic modules are not implemented). Static builds can be done for other platforms of course, I don't think we currently have any such builds in pikefarm though.
Anyway, I think the Pike function combine_path_amigaos() should stay even if native support is (temporarily?) removed as it is part of the API.