Hi, I seem to remember reading something about this button being used to activate a voice operated interface for the touchscreen system (which to my knowledge was never implemented). I obviously can't vouch for this but ut does seem reasonably plausible.
When my GTi was new I tried the buttons on the end of both stalks. Did nothing. Then after the 6 month service, I found that the wiper stalk button now cycles the central display through the trip computer screens, radio, and back to the digital speedo/odo/trip. Maybe the dealer updated something ! But the indicator stalk button still does nothing.
I've spent the last few weeks wondering what the button did on the end of the indicator stalk and from a bit of research it turns out it really does nothing on the lower models. Funny they still allow it to be pressed and haven't blanked it, leading to confusion and frustration!
I was having a play with this on my 2016 GTi Prestige the other day.
I can confirm that, having tested it with 2 different Android smartphones, the button does occasionally connect to Google Now's voice control function, but its main function seems to be crashing the head unit until you turn the car off and on again
on apple car play activates the voice controlas for lower models it cheaper to fit the same stalk rather then have differnt ones for the models.
like most cars have same wiring loom which helps if want to upgrade stuff as it normally there just not in use
I actually tried it on my Active model again the other day while I was sitting on a car park and I held the button down for a few seconds this time. Out of silence the Siri voice suddenly loudly burst onto the car speakers saying that I needed to be connected to the internet. It made me jump!