You can still use it though, but it's not entirely straightforward: you need to sideload it, see https://www.reddit.com/r/apollosideloaded/

Yes you can still use modified versions that allow using your own personal api keys. The reddit api has not changed and a personal api key is free and has something like 1000 hits a day which is plenty even for a heavy user.

https://github.com/Balackburn/Apollo