Seems https://hub.docker.com/u/aarch64/ has moved to https://hub.docker.com/u/arm64v8/ and is actively getting updates from other projects.
MariaDB gained arm64v8 support on 3 August 2018 (https://github.com/docker-library/official-images/pull/4674).
Since then every 10.1+ release has included amd64 support (http://soaphub.org/imagehub/?q=mariadb&n=library&p=1).
Since architecture specific domains for containers where an artifact of lack of poor multiarch support I don't think this is still the case. I haven't seen a user having trouble accessing the mariadb container images for arm64.
With the current high level of multiarch tools in the consumption of images adding a separate release location after 2 years doesn't seem to be worth it.
Contrary evidence as to why a separate domain is still useful is welcome.
Yes. We don't have a aarch64 builder at the moment, but we'd like to have one later on. There's a work underway to make MariaDB/MySQL work better on this architecture.