They've also dropped the requirement that vendors must provide a way for users to override secure boot: https://arstechnica.com/information-technology/2015/03/windo...
They know how to boil a frog.
They may have dropped the requirement, but hardware vendors are still allowing loading your own keys. Because, there are more factors acting on this than just Microsoft.
They literally had a requirement that ARM Windows RT devices be not user unlockable. If someone lunges at you with a knife, but fails to stab you, it is not a "slippery slope fallacy" to believe they tried to kill you. What would it take to convince you? Zero remaining unlocked computers on the market?
What distro are you using
https://wiki.ubuntu.com/UEFI/SecureBoot#How_UEFI_Secure_Boot...