Is there some place where community or RDNS project documents the connections made by popular apps?
The usecase it to have a reference when isolating apps and then only whitelisting domains. Or is the only current way is to do it manually for all the apps someone uses?
Nope. You could start one (write a GitHub wiki, for example), if you want to (: Let me know, if you do.
Yes.
We do want to make this config shareable (github issue). Not priority, however.
Sure, would be great to contribute. The only issues with collection of connections would be “how long do I watch it for” and “how can I trust the provided whitelist”.
The first would probably need covering connection over time, over locations (do connections differ depending on location?) AND over events (post install, setup, update, etc.).
The second would be maybe sharing public logs? Lower prio than first issue. Let me try to figure it out, but it will be slow because of the day job 
Have been slowly moving to RDNS instead of native VPN app, provides so much more granular control. Plus I see something about RPN proxy in the future too, which might be better for me than VPN in some cases.
Our aim with this is to provide a super-cheap anti-censorship proxy. Including being able to connect via Androids of other donors. Let’s see. It isn’t easy but even though we’ve made progress, a lot remains to be done.
There’s bound to be problems with IP-based collections, but domain-based should be okay. Though, in a few cases like YouTube / Instagram / WhatsApp, even domains the app connects are geo-dependent. Perhaps, ASN information will help (we plan to integrate ASN in DNS/Network logs, in the next version).