I have a DBAN CD )used with an external optical drive) which I have used in the past on multiple old Macs. I tried to use it on a 2018 Macbook Pro and DBAN could not be booted. Specifically, I tried holding down the "c" key and I could hear the CD spin a lot but then the normal OS booted. I tried booting while holding down "Option" but DBAN wasn't in the list of bootable options. I tried booting into Recovery and selecting the DBAN CD as the startup disk but I can't choose it (I see the DBAN CD but when I choose it I receive a message that the startup disk can't be used because "the bless tool was unable to set the current boot disk"). Again, if I open Terminal in Recovery Mode and try to bless the CD then it fails.
I wanted to know if this was a problem unique to one Mac so I tested the same CD in an old Macbook Pro (I think 2014 or 2015) and a different 2018 Macbook Pro. The CD is bootable normally in the old one and not in that second new one I tested.
Did Apple make a firmware change in newer systems that could have this effect?