It absolutely does, on both the game's official website (as I quoted above) so that you know in advance and in game on the Race Information screen.
That race had a mandatory pitstop, as indicated.
You wouldn't, unless you are required to. Mandatory use of two different tyre grades is incredibly common in motorsports, up and down the ladder. Mandatory pit stops without tyre change/refuel/driver change are more common in virtual racing; the "one-litre" stop (pitting and taking on one litre of fuel, whether you need it or not) is routine in GT World Challenge esports events.
In this case you missed a mandatory pit stop; you wouldn't need to either change tyres or refuel, just enter the pits, stop in the box, and exit again.