XDA thread:
https://forum.xda-developers.com/android/apps-games/app-migrate-custom-rom-migration-tool-t3862763/post78060823#post78060823

Reddit
https://www.reddit.com/r/Android/comments/9ud6mb/migrate_custom_rom_migration_tool/

Backup:
1. All apps
2. All app data
3. All app permissions
4. SMS messages
5. Call logs
6. Contacts
7. Screen DPI
8. Default keyboard option.

... and restore in your new ROM in a unique way.

First backup everything. Then flash your new ROM. Now (surprise) flash the backup zip file(s)! Yes a part of the restoration process occurs via TWRP recovery. Make sure to root your new custom ROM with Magisk.

Once everything is flashed boot up your new ROM. You'll get a notification to continue the restoration process. Grant root access and relax until everything is restored.

THIS APP IS STILL IN ITS INFANCY. HENCE THERE CAN BE ERRORS. PLEASE HELP IN MAKING THE APP BETTER BY REPORTING UNINTENDED BEHAVIOUR VIA EMAIL.