How do I return to the previous macOS from Ventura? Ventura killed a mission-critical app.

To reinstall the original, shut off the computer, then start it while holding SHIFT + CMD + OPT + R. Choose "Reinstall macOS", which should offer the earliest version your Mac can use. Then, download Monterey with the link in this web page.

Hello,

You can download and install a previous version of macOS from the below article.

How to download macOS - Apple Support

"Or use the App Store or your browser

When you use these methods instead of Software Update, your Mac might need more storage space to download macOS and complete the installation.

Ventura 13

Monterey 12

Big Sur 11

Catalina 10.15

Mojave 10.14

High Sierra 10.13

1. Click the link to open the App Store page for that macOS. 
2. In the App Store, click the Get button to begin downloading the macOS installer.
3. After downloading to your Applications folder, the installer opens automatically. Follow the onscreen instructions to install macOS."

I hope this helps.

Scorched Earth Method 

This method will WIPE ALL Data and there is No Recovery - Period.

>> Only works on Intel Based Apple Computer <<

To perform this action will require booting from a Bootable Installer. 

The Bootable Installer can Only be performed on an Apple Computer 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.

Alternative is, to gain access to a Qualifying Apple Computer from a family member, friend or associate.

Extra Special Notation regarding the Touch ID equipped Apple Computer.

 About Startup Security Utility and Must Enable from Recovery Mode the ability to boot from External Drive Before Attempting 

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map. This applies to macOS 10.14 Mojave and above. 

5A - Formatting for macOS 10.13 High Sierra and below requires HFS Journaled with the GUID Partition Map

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

9 - Then the User can use the existing TM BAckup to Restore the User Account there after

Did you check with the vendor for that mission-critical app and see if they have a newer version compatible with Ventura.

If you had a last Time Machine backup that was not overwritten by Ventura with its first backup, and the previous operating system was formatted as APFS, then you could boot into Recovery, and select Restore from Time Machine backup.

That should repave the startup drive with your previous operating system and user data just as it was before the Ventura upgrade. If you have no TM backup available from the previous operating system, then any installation of a prior operating system would wipe your current drive and user data.

Space shouldn’t be an issue at all. In order to reinstall an older operating system, you will have to wipe the entire hard drive. Then reinstall the old operating system. Then restore your data from before you upgraded to Ventura.

Since your mission critical app is gone for good, I recommend a different approach. You can use a virtual machine to install a copy of Monterey and then reinstall your app on that. You won’t have to wipe the hard drive and you won’t lose any data. You will have time to find some alternative to this app.

I strongly recommend using Parallels downloaded from the Mac App Store for the VM.