Warning! Important information for Macs with the T2 security chip
Before you try to use a bootable installer on your Mac if you have a 2018 or newer MacBook Pro, MacBook Air, or Mac mini, you may need to make sure to allow booting from external media before you attempt to downgrade. If you've enabled the Startup Security Utility, check to make sure you'll be able to use a bootable external drive

Read Here :
https://www.imore.com/how-downgrade-macos#t2

This video will show you how to downgrade from macOS Catalina Public Beta to macOS Mojave including tutorial how to make bootable drive.

This is my first video, I try to downgrade my MBP. I thinks its good to film it :)

Do not forget to backup your important files before downgrading :)

Feel free to contact me if you need help :
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Links :

OSXDaily : http://osxdaily.com/2018/09/26/make-macos-mojave-boot-usb-installer/

MacOS Mojave on App Store : https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12