Welcome to the community!
Super Productivity looks, well, super nice! 
As one of the maintainers of remoteStorage.js, I think I can answer your questions.
Every now and then, someone submits RS, or a remoteStorage.js release, to Hacker News, Lobsters, and such. Usually, there are some good discussions underneath, and more people have it on their radar afterwards.
I had planned to write a longer-form post about the overall status of the protocol, including a request for contributions, but procrastination is a monster sometimes. I hope to get that done soon. (Also, @michielbdejong asked me privately to join as a spec editor, because he is working on SOLID now. So I didn’t want to write some big post until that is official.)
remoteStorage.js is actively maintained for critical bugs. And from time to time there are even new contributors showing up randomly and helping out, when they fix something not just for their own code, but sharing it with the upstream repos.
The decline in commit activity can be attributed to the library being pretty stable as is, and nobody adding new features at the moment. However, we did actually publish a release of the connect widget add-on, with quite some improvements recently for example.
The next big thing some of us want to add is support for partial/resumable uploads, which has to be prototyped both in a client and a server, as it would be a new spec addition.
Personally, I’m also planning to finish my TypeScript branch and propose switching the codebase to that. I think this will make contributions much easier, by making the code easier to understand and reason about, especially for the sync- and caching-related parts.
I hope this helped with answering the immediate questions. And maybe someone sees this and wants to help out with any of the things I just mentioned? I’d be happy to assist in any way I can.