This is
a solvent mixture sold in many hardware stores in the Netherlands, originally as a thinner for Bison contact cement but then rebranded as a degreasing agent (
ontvetter literally means "degreaser", not "stain remover"). Regardless, it dissolves polystyrene so you can glue model kit parts with it. I've been using this for over thirty years after it was recommended to me by someone at a model club meeting.
A 250 ml tin costs about the same as a bottle of liquid model cement, and I just refill a bottle of liquid cement with it every time the brush won't reach the "cement" in it anymore

Well, it is kind of hard on the brush, or maybe that's just because I use the brush for much longer than it's designed for, so every decade or so I buy a new bottle of liquid cement.
Aside from that, a few years ago I also bought a bottle of Tamiya extra thin, to see how well that worked, and as it's much slower than the above,¹ I've decided to keep using it. However, being a cheapskate in some ways I just purchased 100 ml bottles of its two ingredients plus a 10 ml lab pipette and refill that glue bottle, too, when it runs low
¹ I know, most people think TET is quick, but I use it when I need glue that doesn't evaporate as quickly as my usual type …