catalina downgrade

Downgrading isn't easy unless you have a clone or Time Machine backup. Do you have one of those?

Revert your Mac to a previous macOS version [with Time Machine] - https://support.apple.com/guide/mac-help/mh15216/mac

https://communities.apple.com/fr/thread/250788825?answerId=251498564722#251498564722

"It is possible to attempt to return to the old system before installation even without Time Machine backup. Mac Os sets up an internal backup system (Snapshots) on special occasions and the system update is one of them.

In short, it is necessary to start in recovery mode (cmd + r during the boot) then to request the restoration Time Machine. There select the system disk (Macintosh HD in general) as Time Machine source and see what is on offer. With a little luck will appear a date / time prior to the update with the correct system version. If so, select it and then start the restore." [Google Translation]

Is there a reason behind your request, that additional information might resolve — without the disruption of retreating the operating system?

See if newer versions are available, but yes, Catalina is resulting in the mass extinction of older software.