This is not a recognized method but if you can get access to your own account on a computer with the required minimum macOS version for the current version you can purchase/buy the app on it. Now go to your old computer and bring up _your list of previous purchases_ (the previous purchases part is important - see the tip at How can I get Garage Band for OS 10.14? - Apple Community ) in the Mac App Store application. Try downloading it. It will say the current version will not work on your computer but may offer an older version if one is available.
If this is an app that interacts with a service offered by the developer (e.g., YouTube or Netflix) there is no guarantee the old version will actually work with the service, but I have found some old versions are available and provide partial functionality.
Despite what may or may not eventually be recommended for you to read, I know this works in June 2023, and has for several years now. in 2019 I had access to a Mac running macOS 10.15. I downloaded Pages on it and was able to get an older version from my purchase history in the Mac App Store on another computer with macOS 10.9. In 2023 I was given a 2013 Mac Book Pro running macOS 11. I went to the App Store and it said the current version required at least MacOS 12 to run, did I want to download an older version? I said "yes" and it downloaded the version compatible with macOS 11.
June 2023 thread about downloading older apps on a Mac.
How can I get Garage Band for OS 10.14? - Apple Community