No, because it is still x86-64, it just bypasses the bootstrapping process that exists right now going from 16 bit to 32 bit to 64 bit by starting the CPU in 64 bit mode.

However 32 bit registers and everything will continue to exist, and it will act like todays CPU's that went through the multiple bootstrap rounds to get into 64 bit mode.

Which begs the question as to what 'support' linux needs given that I believe EFI does the job of entering protected mode these days.

EFI only spins up a single core/thread. Starting up the other cores/threads is different than it is now.

To rephrase your question, are there any Long Mode user space changes?

Skimming Intel's spec ( https://cdrdv2.intel.com/v1/dl/getContent/776648 ), the X86S looks really risk-adverse towards changing Long Mode user space.

I don't recognize any changes offhand, but my knowledge of x86 legacy is pretty limited (and will hopefully be obsolete soon).