I just tried booting Debian 10 Buster with a backported kernel v5.7 and it did not work well for a T2 MBPro 2019. The external Debian drive booted just fine, but I had no access to the built-in Keyboard, Trackpad, WiFi, or Ethernet. Either macOS support is not included in this kernel or the macOS support was not enabled by default if the macOS support are considered experimental features. At least with a 2017 MBPro I did have working WiFi.
Here is an OWC blog entry that mentions the error you are seeing and it is associated with macOS and T2 security itself and should have nothing to do with Linux (since I believe this occurs when re-enabling "Full Security":
https://blog.macsales.com/46677-all-about-the-apple-t2-security-chip-and-startup-security-utility/
How Do I Enable Secure Boot?
The three settings available for Secure Boot are Full Security, Medium Security and No Security.
Full Security
Full Security provides the same level of security as iOS devices, and it is the default setting for Secure Boot. As the Mac starts up, it verifies the integrity of the operating system on the startup disk to ensure that it is legitimate. If the OS is either unknown or not verified as legitimate, the Mac connects to Apple to download the information it needs to verify the OS. That information is unique to each Mac and is used to make sure that the Mac is starting up from an OS that is trusted by Apple.
An internet connection is required for verification of an unknown or non-legitimate operating system, so make sure that the Mac is connected to a Wi-Fi network or Ethernet.
If the operating system doesn’t pass verification, the following happens:
macOS: The system alerts you that a software update is required to use the startup disk. Clicking Update opens the macOS installer, which can then be used to reinstall macOS on the startup disk. The other option is to click Startup Disk and select a different startup disk, which the Mac then attempts to verify.
FYI, I did have trouble re-enabling "Full Security" since macOS Security Utility seems to be poorly designed and is not intuitive at all. Apple needs to hire someone who knows how to test software implementations to make sure they work smoothly. Q & A is lacking with the Security Utility.
Is your Mac being managed by a business or school or with FindMy, etc. where maybe you are not allowed to disable "Full Security" mode? I'm not sure you how you can verify within macOS the status of the "Secure Boot" or "Full Security" feature.
