>>antina+(OP)
This is an unfortunate, but not unexpected, end of an era. App.net was created at a time when discontent was high with how Twitter was treating its users and 3rd party app developers. Even though App.net wasn't hugely successful, its existence provided a needed check against Twitter exercising user-hostile control over their platform.
However, it has not been a viable platform (one that people actually used) for many years, so while I am saddened that it is finally being shut down, I'm not surprised. Many thanks to Dalton and everyone who built it and kept it going these many years!