It would be great if the Brightbox Ruby PPAs could provide packages for the arm64 / aarch64 architecture.
This should work fine on Trusty and later and Ruby 1.9.3+.
However since Trusty is EOL and Xenial is EOL in April, adding them to Bionic and Focal should be enough.
I'll be happy to help with testing!
I've already built a few versions for Bionic on arm64 using the NG PPAs source packages and Docker for arm64 on macOS.
I also tried a larger range by installing through RVM, which is how I discovered the lowest aarch64 compatible Ubuntu and Ruby versions.
Regards, Felix