Hi Markus here – from the MusicApps Team at Apple,
I’ve tried to contact as many Audio Unit developers as possible before WWDC to get your contact information. I’ve gotten a lot, but obviously not all. We have started a brand new ProAudio Seed program last week and if you were contacted by me or we already had your contact info, you should have received an invitation to the seed program. Some did not, simply because your email provider considers invite emails from Apple spam. We are not sure, how we can resolve that on our side.
We are seeding universal versions of Logic Pro, MainStage and GarageBand build against the macOS 11 SDK, which should help you significantly to test your Audio Units. The seed program also allows you give direct feedback to the MusicApps team. For Apple Silicon development and testing you need the DTK. But for Big Sur compatibility testing on Intel, the seed is also extremely helpful.
I am sorry, but I am not able to provide support or answer questions here. If you are an Audio Unit developer, please contact me with your website, some of the AUs you are working on, your full name, and preferable the Apple ID email used for your developer account at mfritze@apple.com. I know, that might sound like phishing, but it is not – maybe some people here can back me up on that 
Thanks,
Markus Fritze