Custom filters are absolutely possible, it just depends which adblocker you choose to enable. IIRC the block list has to be compiled to a static JSON file for Safari to consume, but most of the decent adblock apps on iOS provide a companion configuration application that lets you insert custom entries then recompile the block list.

If anything I prefer this way of working - Apple alone are responsible for maintaining a high performance adblocker in Safari, the wider app ecosystem just provides the block lists, rather than rely on app developers to write performant browser plugins.