Premier Crashes After IOS Update

yup.  You'll have to backup everything you'll need

If you decide to roll-back your macOS, these links may be helpful:

How to upgrade to macOS High Sierra

https://support.apple.com/en-us/HT208969

Scroll down to step 4 where it reads, "If you still need macOS High Sierra, use this App Store link: Get macOS High Sierra." and use the link to take you to the macOS High Sierra download in the Mac App Store.  That link allows you to get to the installer for High Sierra whether it's an upgrade or a roll-back.

Or,

How to upgrade to macOS Mojave

https://support.apple.com/en-us/HT210190

Scroll down to step 4 where it reads, "If you still need macOS Mojave, use this App Store link: Get macOS Mojave." and use the link to take you to the macOS High Sierra download in the Mac App Store.  That link allows you to get to the installer for Mojave whether it's an upgrade or a roll-back.

May not be easy to downgrade.  I strongly recommend that you create a bootable clone of your startup drive before doing any major update.  This way you can restore your system to the previous OS without too much heartache.  I use a program called Carbon Copy Cloner and it's worth the price in my opinion.     https://bombich.com/

You can also use a timemachine backup, but I prefer to have the control of a bootable clone.  I'm still working on a macpro which has 4 internal drive bays and my boot drive is on an ssd.  I have a clone on a physical hard drive in another bay so I can quickly boot of the hard drive and get back to work almost immediately.  CarbonCopyCloner also allows you to do an incremental backup, so keeping it up to date doesn't take too long.

Downgrade the OS and wait to install Catalins when Adobe releases updates to Premiere Pro.

Why don't my Adobe apps work in macOS Catalina 10.15?

"However, these apps were not designed or fully tested on macOS Catalina. We recommend that you continue to use the macOS versions noted in the system requirements for each app."