Correct, GarageBand on iOS (and even on macOS) doesn't yet support MIDI Processor type AUv3s. GarageBand for iOS is overdue for a major update, maybe we'll see something in the near future.
Your best bet for testing is Logic Pro on macOS and AUM on iOS.
NikoloziApps
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It's hard to test originalPurchaseDate also, my TestFlight build returns 2013-08-01 and my app has only been available since 2023 May. And I agree with the OP, documentation is confusing. My TestFlight build originalAppversion returns 1.0 for both Mac and iOS. But for the local debug builds I get the current version number 4.8 for macOS and I get the current build number 1 for iOS. And the purchase date is 0001-01-01.