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php-remote-storage is probably the best one for a newcomer, and it's maintained by @fkooman who is a co-editor of the protocol draft.
Could you elaborate why it seems too complex for a newcomer? It would help a lot with fixing that. Also, it's open-source, so any contributions -- even if it's just opening an issue on GitHub -- would help a lot.
mysteryshack works well in general (using it all the time for app development mself). Apparently @untitaker decided to stop maintaining it less than 24 hours ago. So it's still well up-to-date, and as it is open-source, and based on modern Rust code, it's easy for anyone with Rust knowledge to update it from there on.
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restore is a bit of a difficult story, because unfortunately the original author doesn't seem overly interested in coordinating any contribution efforts. @les has been maintaining a fork of it, with the last commit from September 2017, and I just noticed that someone else had also rewritten it in modern JS (according to the last commit message). Check out the network graph.
It would probably make sense to update the wiki in regards to that information, especially because it looks like that last fork is not using @les's improvements.
However, in general that means restore seems to be alive and being worked on, but people are currently being directed to the unmaintained version, and it would be great if we could coordinate development efforts between the new contributors!
I don't know about Indie Hosters really, but I think they were using restore, and afaik it would mostly be a matter of providing a Docker container with anything that works, in case we want them to provide RS again.
liquor-cabinet is actually the open-source RS API part of 5apps Storage, and in active use by tens of thousands of people every day (as the hosted version). It is actively maintained by the 5apps team, and we keep adding fixes and improvements all the time, based on edge cases we see in the wild. The reason it's only the API part, and doesn't contain signup/auth and Webfinger, is that it is meant to be added to existing systems and accounts. So yes, this one is not for end users who self-host, but for any providers who want to add RS to their users' accounts.
I think the Cozy add-on was based on remoteStorage-server, which indeed is kind of dead. Same as rs-serve, which is also a very old project.
So, a the end of the day, for end users right now php-remote-storage and mysteryshack would be the best choices imo, with restore looking like we could coordinate to make it a legit choice as well.
Considering that the RS protocol spec is actually still a draft, it's kind of a miracle that we already had so many different implementations being built by so many different authors. Most pre-RFC drafts are lucky to even have two different ones. And with usable implementations in all of PHP, Rust and JavaScript I think we're in a great position for getting the protocol to a final state sometime soon. Go, people! Go, open source! 
P.S.: I just opened an issue on the latest fork of restore, suggesting to coordinate efforts with @les and maybe create a new shared repo/org, where we can send users and developers.
P.P.S: We already got the ball rolling to move reStore to an org repo under a new name and make it a thing again.
