Can the A20 line still be masked off on Haswell and successors?
It seems there is at least one Skylake platform where theA20 line cannot be disabled anymore.
Apart from my testing, I've picked up the first schematic of a Skylake laptop I could find online (Acer Swift SF314-51):
The thing on the right is the EC, which has an A20GATE connected/pulled up to +3VS.
I'm not really familiar with the symbols in this schematic but those are the only A20GATE traces present, there is no connection to the PCH.
Considering that (according to the datasheets) the PCH has no support for the A20 gate emulation, this seems a piece of strong evidence in favor of the absence of the A20 masking feature on this platform.
This reasoning is valid iif I've correctly understood the schematic and only proves that there exists recent x86 platforms where the A20 masking doesn't exist anymore.
I'll leave this question unanswered until somebody comes with a more complete answer.
Section 188.8.131.52 (External Signal Compatibility) of volume 3 of the Intel SDM (April 2021 edition) says:
The functionality of A20M# is used primarily by older operating systems and not used by modern operating systems. On newer Intel 64 processors, A20M# may be absent.
(emphasis is mine).
It doesn't specify when this was removed, but you can assume that it will be unavailable starting from a certain point.