I have a late 2018 car.
Interestingly, my manual states that if you press the button for 3 seconds- for the gti- TCS is off but ESC goes to sport. As you say, for the golf R, ESC can be switched off ( according to the manual)
Just checked my car- contrary to info in the manual, in fact I can turn ESC “off” with a 3 second press.
It’s been a while since I tried this so I’m not sure if this completely disables the stability systems- I suspect it would kick in if I completely lost it.
I’ll try it out one day.