MariaDB-backup RPM packages are available for current versions via the download server on mariadb.org and its mirrors, and they are also there for older versions on the archive.mariadb.org server.
The foundation download site does only provide access to the archive server via the hosteurope.de mirror though, and on that the MariaDB-backup packages are missing somehow.
Compare directory listing on
to the mirrors listing:
PS: hosteurope mirror for current version downloads is fine, this only affects the archive mirror. This also not only affects 10.3.21, but apparently all archived versions.
PPS: mariadb.com download server is also not affected, only the foundation archive mirror