We do not have a list of "up to" supported versions.
We test on latest "production" PHP version at the time of the release.
Sometime we mention in the release the latest version we tested on
http://www.abantecart.com/shopping-cart-newsAs for 1.2.11, I believe it was tested on 7.1 as we did that prior to that version.
Hopefully, we will finish unit tests for entire AbanteCart soon, that way we can automate testing on multiple versions.
We need community help for that.