is there any conclusive answer about whether you can natively boot/run Linux or Windows on a T2-equipped Mac Mini.
Apple has a Whitepaper on the T2 linked off the Mini's and MBA's page.
Apple T2 Security chip overview (PDF file).
There two pragmatic preconditions before it will do much of anything.
1. You are required to have a admin account on a macOS instances. ( Essentially this is the default "owner' of the Mac if simply just start it up and use it. ). Adjusting the basic boot/authentication security of the Mac depends upon this. Long term that probably is not going away. You'll need some instance of macOS to stick around over the long term. (even after Apple may drop support you'll need a small instance as least as utility tool. )
2. Pragmatically you need to run Boot Camp assistant. ( and it is dependent upon precondition #1 being done. ). Simply turning off the boot os validation setting is technically not a "boot anything" mode. It is more like boot anything macOS. Running Boot Camp assistant opens that "anything" up to at least Windows.
Apple will not do a UEFI like 'Secure Boot' of a Microsoft signed Linux instance. The T2 trusts Microsoft about Microsoft product (Windows), but doesn't trust Microsoft validation of other organizations' code.
It is fuzzy in the whitepaper about whether this opens up a gap for Linux to go through, but probably has some other side effect than simply just Window's bootloader.
3. the Boot (or not ) external drive and OS validation settings of Boot Security are independent dimensions. Turn the OS boot validation doesn't pragmatically "turn on" external boot. To get to Linux you'd have to crank both dimensions all the way down and run BootCamp Assistant. All of that.
4. Pragmatically right now you'll need a drive that isn't the T2 drives ( effectively an external drive on all the Macs so far since they are all one drive only instances ). Right now linux can't 'see' the T2 drive. So effectively an external drive ( hence why need to crank down all the security settings )
In short, you can't just "raw disk image" a new Mac into a Windows or Linux box. There can be other OS present, but as a Mac there needs to be macOS present in some form.
What is the driver situation?
The 'can't see T2' might clean up with a future driver, but it may also be a security measure (e.g., if something on the other side doesn't 'smell' like macOS just stop talking to it. ). That may also be some bootloader mods necessary to ease how complicated switching to/from booting macOS is.