I'm intending to release the current master on github as version 5.9 in the near future. Could I ask everyone to give it a try and let me know if there are any serious bugs that need to be fixed. The main differences are the ARM64 code-generator, the new bootstrap process and position-independent code. This was described in greater detail back in May http://lists.inf.ed.ac.uk/pipermail/polyml/2021-May/002451.html . There have also been other smaller changes and fixes.
David