From @Cherenkov11’s instructions in this post:
$ mkdir rtd3 && cd rtd3
$ sudo pacman -S acpica
$ sudo acpidump -b
$ sudo chown $USER:$USER ./*
$ iasl *
$ rm *.dat
I then grepped for _PR3 and _PR0:
$ grep -ir "_PR3\|_PR0" ./*
./ssdt3.dsl: Name (_PR0, Package (0x01) // _PR0: Power Resources for D0
./ssdt3.dsl: Name (_PR3, Package (0x01) // _PR3: Power Resources for D3hot
./ssdt3.dsl: Name (_PR0, Package (0x01) // _PR0: Power Resources for D0
./ssdt3.dsl: Name (_PR3, Package (0x01) // _PR3: Power Resources for D3hot
./ssdt6.dsl: Name (_PR0, Package (0x01) // _PR0: Power Resources for D0
./ssdt6.dsl: Name (_PR3, Package (0x01) // _PR3: Power Resources for D3hot
So it looks like Runtime D3 is supported (I think - need confirmation).