Good on the Lux team for making it optional, but this is yet another app that wouldn’t have needed to introduce a subscription model if Apple would just relent on its unbending services push and add a mechanism for paid upgrades to the App Store.
A bit more expanded at http://www.eiman.tv/blog/posts/upgradepricing/index.html - haven't looked at this since 2016 though so perhaps something has changed since then.