Hi everybody. This comes up regularly. Here is the current state of ARA in REAPER.
1. We worked with Celemony when they first introduced ARA in 2011. They are helpful and nice.
2. At that time, we implemented the REAPER AudioAccessor API, which provides access to pre-FX audio at either the track or media item take level. This is essentially an implementation of ARA, but without the ARA interface. That API has been available in REAPER since 2011, and some REAPER users/scripters use it for various interesting ARA-like purposes.
3. Once we got to that point, where audio data could be accessed on demand by any external code, we then did some work to implement ARA on top of our API. But the complete ARA interface requires a significant amount of code overhead. Since we already had an audio data retrieval implementation, we chose not to spend the additional resource cost of developing and continuing to support the complete ARA interface. To be clear, "resource cost" simply means our time. This is not a money issue.
4. In 2016 we opened up the ARA API again and did some more work, but again reached the same conclusion.
What stopped us both times was the specificity of the work. It's easy to justify spending days or weeks on a new feature that is generally useful, and harder to justify spending the time to support essentially one plugin.
Over time several technical users have proposed building a third-party bridge between ARA and our AudioAccessor API, which is not technically impossible, but it would be an extremely complex undertaking.
In conclusion --
We understand that some users really, really want this. It's not really feasible as a third-party project. We understand the benefits and we also understand the development costs. We may at some point finish the project.