If the mods work with PHP 5.6, then chances are they work with 7.0 as well. 7.1 is where you're going to run into a few potential issues, and I've no experience with 7.2 yet so I can't comment on that.
As PHP7 offers such an enormous advantage, most people who have a choice will use PHP7. Therefore I doubt there will be more than a few (if any) addons that dont work with php7. php7 was released almost 2 years ago and xf is fully compatible with it. If an addon is not compatible with php7 by now then its not well supported anyway.