OS X 10.15 (Catalina) is supported as of Authentication services version 4.2.2.
This can be downloaded from the following link:
Please see the Authentication services Release notes for a list of all supported platforms.
However note that the 4.2.2 release notes do not currently show OS X 10.15 as a supported platform.
The release notes will soon be updated to correctly show this as a supported Operating system.