Multiple vulnerabilities (CVE-2022-{23803,23804,23946,23947}) 5.X series is masked to avoid accidental use, but it's kept for industrial users who are in the process of migrating to the new format of the 6.X series.
The KiCad symbol libraries are the individual .lib files, with the
corresponding .dcm files containing symbol metadata. These symbols are
best used in combination with the official footprint libs.