Fossify Gallery on the official F-Droid repo
The removal isn’t directly related to the buyout/fork. Simple Gallery was taken off of F-Droid due to a dependency on the nonfree Google VR being discovered by IzzySoft¹ ². Fossify’s fork has removed the dependent features to be compliant³.
Their point regarding old and unmaintained apps seems a bit silly. Getting the same old and unmaintained app from the app’s website isn’t going to make it any safer. You’re still going to need to switch to a different app/fork to get updates.