You can temporarily enable the devel repository and that package will be available. (It’s not recommended to keep it permanently enabled)
[label@awx ~]$ sudo dnf info freeradius-mysql --enablerepo=devel
Last metadata expiration check: 0:02:24 ago on Thu 29 Sep 2022 01:22:06 AM MST.
Available Packages
Name : freeradius-mysql
Version : 3.0.21
Release : 26.el9
Architecture : x86_64
Size : 34 k
Source : freeradius-3.0.21-26.el9.src.rpm
Repository : devel
Summary : MySQL support for freeradius
URL : http://www.freeradius.org/
License : GPLv2+ and LGPLv2+
Description : This plugin provides the MySQL support for the FreeRADIUS server project.
The reason why freeradius-mysql is not part of any repository is because it is not shipped by Red Hat. If they decide to add freeradius-mysql to AppStream or CRB in the future, then it will appear in our repositories as well.