Intertrack spacing and decoupling cap package for high voltage buck regulator
If you look at the LM5085 data sheet, Vin is on pin 8, the corner pin. So it has only one adjacent pin, which is Vcc pin (pin 7). This is normally at 7.5V below Vin which is 68V for 75V in. The remaining two pins on that side are Pgate and Isens.
Psens does between Vin and Vcc, and Isens is normally connected to a sens resistor on the external FET.
So it seems that TI thought of this - the voltages between adjacent pins on that side of the chip are nowhere near 75V, even with 75V in.