Event details
We have several questions
First, is there any risk that devices could become unbootable if they have not been regularly updated with Windows Updates up to no
If you ask that broadly, the answer would be YES, but totally unrelated to Secure Boot. When machines exposed to the Internet are not up to date, they can catch malware. Some poorly written malware can result in the machine getting unbootable as an unintended consequence, and some malware/randomware deliberately prevent your machine from booting.
But none of the Secure Boot expiration times will directly make any machine unbootable.