How are you supposed to test Mac apps that support Mac OS earlier than 12?

Our QA team is trying to test our application on all platforms we support, but Apple has inexplicably blocked installation of TestFlight on OS 11.

Does anyone know the rationale behind this, or how we're supposed to distribute test builds in an orderly manner now? Having to make ad-hoc builds piecemeal and E-mail them around with duplicated instructions is not exactly professional.

Thanks for any insight!