Garageband for Catalina 10.15.7

We can provide telephone numbers and Apple Websites to communicate with Apple regarding this issue.

But even they, may not be able to help in getting an older version GarageBand available for this computer

As you have discovered, painfully so.

Without a previously downloaded version under Your Apple Account ( formerly Apple ID )

It maybe virtually impossible, sadly

Start now and open an Apple Support Ticket as they are PAID  Apple Employees to deal will these types of issues . 

 Product Feedback - and make it known to Apple regarding this ongoing issue

call 1 800 MyApple

Other location Call Centres 

 Contact Apple for support and service

FWIW, there is one possible way for you to get an older version of Garageband. If you know someone who has a Mac running Ventura 13.5 or later. You could log into the App Store on their Mac using your Apple ID. Download Garageband to their Mac. They can always uninstall it afterwords. Then on your older Mac, you could search for Garageband on the App Store. When you try to download it, it will ask if you want to download an older version.

Apple is the developer but I'm afraid sending feedback will not change anything.

None the less: Feedback - GarageBand - Apple