Why solder mask are not applied to RF PCBs
There are several reasons.

1) Soldermask is lossy, and different types of mask are differently lossy. So having no soldermask where the RF fields are gives the best transmission, and if your board is made by different fabs, the most repeatable transmission.

2) Line dimensions, which affect characteristic impedance, are critical. It's difficult to optically inspect them if they're covered with resist.

3) In development, you might just want to add an attenuator pad, or pickoff resistor, to the line. This is tricky enough as it is, without having to start by scraping resist off.