Actually, we've updated several ModX Evolutions from 1.0.15 to 1.2.1, because providers are cancelling PHP < 5.6 bit by bit. 1.2.1 runs on PHP 7.1, if you don't run older Snippets/Plugins. But you've written a good fix for Maxigallery, which we're using for PHP 7.1: https://forums.modx.com/thread/99731/evolution-1-1-maxigallery-upload-picture-button-magage-pictures-don-t-show-up
We've encountered some site-specific compatibility issues, but they can all be solved.
In general, 1.2.1 doesn't work with PHP 7.2. Mainly, you'll encounter "countable"-errors (-> Google), the causes are buried deep inside the code; IMHO no chance to fix. Some providers suppress those "countable"-errors or handle them with a lower error level. There, ModX 1.2.1 works, but I wouldn't count on it.