Given the length of time since 4.8 became stable and the fact no one is working on the ICU stuff, I think it would be best to move forward with releasing 4.8 with a Known Issues addendum that encourages others to contribute if they'd like things like a complete ICU.
Lucene itself has advanced several major versions in the meantime, to 7.6, and catching up with them to reach better API parity seems like a better use of any core dev resources than trying to perfect something 3 major versions old.
