From what I understand this does not prevent use of custom ROMs, it just means ROMs built before it was done will not work anymore. I assume they can re-package old versions to work with the new configuration, I am not entirely sure though. There are discussions elsewhere in this thread with more informed people.
userbinator|1 month ago
From the article:
Any subsequent attempt to install older firmware results in a permanent "hard brick" - the device becomes unusable
This implies that not only does an older custom ROM not work, but neither does attempting to recover by installing a newer ROM.
dcdc123|1 month ago