The spec drafts expire every 6 months. With the spec drafts being published as IETF Internet Drafts, it is generally discouraged to even run RS in production at scale for any company, even though at the same time it can only become an RFC, when there are multiple implementations available. The point is that drafts are not considered stable at all, and an expired draft is not really the same as an older spec version.
So yes, if e.g. nextCloud were to officially support RS before it’s an RFC, then they would have to implement the final RFC while still supporting expired draft versions. As there’s no telling what would be changed in the spec once we hand it to the reviewers, there could be critical, potentially breaking or incompatible changes that would even make this impossible. So I think it is critical that we push the RFC process forward asap if we want mass adoption.
You can find more information about both the IETF process, as well as our discussion around “finishing” the spec in this topic: The road to RFC. Or elsewhere?
TL;DR of the latest status is that we still wanted support for resumable uploads, because single HTTP PUTs are just not enough to upload large images, longer audio, or video files, especially on mobile connections. That discussion is always open, and also the topic of resumable uploads can be picked up by anyone who wants to prototype that functionality (for example with rs.js and armadietto), so we can find out what to spec exactly. The leg work around how this could work has already been done!