No way around that and it’s very normal for Apple/IOS apps.
I had an old iPad like you that I used with djay2 for years on iOS 8. When I decided I wanted to upgrade I found that the new Djay needed minimum of 12+ to make use of the new features & my iPad would only update to 10.3.
I bought a new IPad Pro because I needed the new features, the old iPad wasn’t fast enough, I couldn’t upgrade most of my other apps for the same reason & djay2 has issues with searching the library when you type the 1st letter and this will never now be fixed.
But your upgrade is linked to your Apple ID so go get a newer iPad that’s capable of running the new Djay & the apps you won’t have been able to update for a while.
Go into the App Store, look in “Purchased”
Find Djay 2 or whichever old version you used before, and install it.
Now when you run the app it will tell you about the free upgrade and it will take you to the App Store to download the new version which will unlock some of the subscription features and work fine. (Unless they ever take the free option away!!)
Ps. don’t go crying about an old iPad. I have 5 iPhones and one by one Apple made sure you can’t use the old ones for different reasons that required buying a new one. Blame apple not Algorridim.