Interesting I was just looking at the Chez Scheme backends and didn't remember seeing aarch64, just 32 bit arm. And then I found this https://github.com/cisco/ChezScheme/issues/545 Racket-Chez is different from Cisco-Chez by > 600 commits. I wonder what the plan is to normalize them or will be Racket-Chez be our new house. https://github.com/racket/ChezScheme
I don't think it's likely that we'll move back to pure upstream Chez in the near future, but it's not quite a fork either.
You can read more about some of these issues in our ICFP 2019 paper: https://www.cs.utah.edu/plt/rkt-on-chez/ which is co-authored with the Chez maintainers.