The latest version of the Analytics Designer Professional (20.4) is supported on Mac OS 10.11.6, 10.12, and 10.13. New product releases of the designer occur quarterly. If you have an OpenText support plan, you can open a support ticket and request that support for "Big Sur" be added to an upcoming version. If you are running OS BIRT, please refer to the release notes for your version.
You can install the designer on an unsupported OS, but be aware that you could encounter some issues. Also note that if you have a support plan, the OpenText support team will not be able to assist with an unsupported version.