There’s been a recent spate of T2-equipped Macs which have suddenly become bricked and unusable. One cause for this has come to light: trying to downgrade the firmware in that Mac, for example when trying to restore Mojave on a Mac which has been running Catalina beta, according to recent tweeted reports by @tperfitt and @eholtam.
One route to bricking your T2 Mac is trying to restore an older version of BridgeOS, which runs on the T2, using Apple Configurator 2. This is a recommended procedure for dealing with corrupt or damaged T2 firmware. I have explained it here, further practical details are given by Mr. Macintosh, and the latest Configurator Help pages are here for the iMac Pro and mini, and here for MacBook Pro and Air models.
@eholtam warns that attempting to restore BridgeOS on a Mac running a beta-release of the firmware, which would therefore try to restore the current (older) release version, results in the following fatal error:
AMRestoreErrorDomain error 79 - backwards update not allowed: 17P50541b -> 16PXXX)
This renders that Mac unusable, and it must be sent to an official Service Provider for repair.
Normally, when you revert to an older version of macOS, firmware shouldn’t be downgraded to a previous release, something which should be avoided by the installer. However, some users have reported that when downgrading from a beta release with beta firmware, a similar problem can occur and the older BridgeOS fails to install, leaving that Mac unusable.
If this happens, the only method available to try to fix the firmware problem is firmware restore using Apple Configurator 2, which is also doomed to fail, and that Mac has to go for repair.
If you have a T2-equipped Mac on which you run beta releases of macOS, be careful to avoid downgrading from that beta, if the BridgeOS version differs from the current release. One workaround which appears safer is to run the beta from an external drive, and keep the release macOS on internal storage. When you want to revert to that older release version of macOS, you should then be able to restart from the internal disk and eject the external drive. However, attempting to restore the firmware then could still result in that Mac becoming bricked.
Apple needs to address this serious problem, either by allowing downgrading of BridgeOS, or by ensuring that the version of BridgeOS delivered to a Mac is never older than that already installed.
As Eric kindly points out in his comment below, with the release of macOS 10.15 Catalina and its new firmware updates, it should be possible now to unbrick those Macs which have become stuck unable to downgrade firmware installed by a Catalina beta. I wish those affected every success, and hope their Macs now spring back to life.