“The reason is a combination of Google making Play publishing something between hard and impossible and no active maintenance. The app saw no significant development for a long time and without Play releases I do no longer see enough benefit and/or have enough motivation to keep up the ongoing maintenance an app requires even without doing much, if any, changes.”
I’ve been using Syncthing-Fork for years. It’s better all around because it moves sync conditions to within each sync folder.
So my photos sync immediately (over any network or battery status), but my app config export folder (say my podcast app) only syncs on power and wifi.