Custom keyboards - on iPhone, long-pressing the mic button in Gboard has to run as a separate app instead of integrating directly into the keyboard. Also on iPhone "secure" screens (that use logins and passwords) never yield to 3rd party keyboards; instead they always revert back to the iPhone keyboard.

Browser plugins - as you pointed out, browsers on iPhone are not allowed to run plugins, so no ad blockers in the traditional sense - you can install a system-wide ad-block engine.