I haven’t opened Safari recently but I’m pretty sure Safari in general just doesn’t have user defined search engine lists at all on any platform and never has. There’s ways to hack it with extensions, but otherwise if you want to switch SEs, you can use one on Apple’s list or you switch to a browser with more flexibility on iPhone, Mac or iPad.

Of course since one of the options in Safari is DuckDuckGo, you can also use its extensive list of !bang operators.