Aeon Fossify are a Fork of the old Simple Mobile Tools from before they got sold to an Ad tech company. One of the Devs made a Fork to keep it Open Source and Ad free. If you have used SMT Fossify is its successor.
N1b Yes as of now you should probably replace any SMT app you use with the Fossify version (or another up to date open source app not owned by an ad company). I do use calendar and gallery from Fossify, they're good.