My phone broke last week so I bought an iPhone 11.
My Mac wouldn't connect/sync with it unless I upgraded to Catalina.
So I upgraded to Catalina, and now all of the software I used on Mac doesn't work at all. (iZotope Audio editing, Native Instruments, Cubase, Audacity, and more. None of it works in Catalina.) Catalina also makes my Mac Mini not recognize my LG monitor unless I Cmd-Opt-P-R flash PRAM and reboot a few times just to get it to send a signal to the monitor.
So now the Mac is useless to me. I literally can't use it for anything I was using it for. It has no working programs on it.
So I switched back to Android, found an old Windows laptop, re-installed my audio software on Windows, and I'll just work without Apple until they fix their OS, or maybe never again.
1. Use some mac to download an older OS installer. The links are hidden but you can find them in some forums etc.
2. Create a bootstick - either manually or with software like dosdude's really nice patchers (find them here: http://dosdude1.com/)
3. Backup your stuff (not only time machine as it would reinstall the new version of macos again) and install the old macOS to get your apps and files back on it
This is something I really hate about Apple - why can't they just provide some official download links that keep working?
With Apple I have a love-hate-relationship because some products (MBP until 2016/17, iPhone 5s/SE) and software (keynote, pages, lot of parts of the OS) are awesome while their software policy and planned obsolescence screws you hard if you just follow their lead as a simple user.
You always have to take care not to update too early, use workarounds to integrate devices with different Apple software (like with airdrop) and if you brick devices there is few ways getting back to business (no helpful error messages like the icons they show on boot when something goes wrong or messages from their system diagnostic like "HDD is damaged" while it's just the cable).
The tool from dosdude can do a bit more. With it I managed to install High Sierra on a MBP from 2009 (which isn't compatible anymore but it runs flawlessly).
Before it had an HDD with Windows only and no access to recovery mode or diagnostic tools. Thanks to the "High Sierra Patcher" everything works now :)
You can sometimes finagle it by playing around with kexts (I once even had success with a custom kernel designed for AMD Hackintosh’s), but it’s a somewhat involved process with no guarantee of success.
You really don't have 'complete control' of Macs any more. Apple do what they like with your hardware.
Been like this for many years now.