To be honest, if add-on devs push the boundaries a little bit above our minimum then it likely helps us adopt newer versions quicker. So I'd probably say 7.1 or 7.2 is reasonable.
PHP 7.2 does allow for return type hinting objects...
I think a reasonable compromise would be to add a warning on install/upgrade of my XF 2.1 add-ons to let people know that they should post feedback @ my site if they are unable to upgrade to 7.2.