Is this illegal?

[Dum-Dum]

Untill you can get measurments no one can really comment yes or no. All I can say safly is if I saw that on mine I would replace it

No you can, Tyres have built in wear markers at 1.6mm and that tyre is clearly worn down to the level of the markers so you can say yes its illegal.

Look at the right one of the 2 main circumferential tread bands and at the bottom of the picture and you can see the marker.