Horizon is a massively open world game, the draw distances and environments in the trailer were incredible. RT would have been incredibly expensive outside of some real shoddy implementation like WD Legion's.
GT 7 will have checker board RT but that's a strictly track based game. I imagine the next Forza Motorsport will also have RT.
Tbh, a dense urban setting with lots of reflective surfaces like in Spide-man Remastered / Miles Morales or Watch Dogs Legion would be more taxing than one set across the countryside, as you'd need to include more dynamic objects in the BVH. In FH5, the cars themselves and occasional wet tracks, puddles, and river streams are the only major reflective surfaces.
Forza Vista is basically a real-time photo mode where you can pose your car and driver avatar in scenery locations in the world to take pictures/videos.
Even without RT the game looks fantastic. Plus given Microsoft's support for first party games I am positive that the RT mode might come as an update after the release similar to Spiderman's RT Performance update.
No game changing RT reflections for Necrokiller. No seeing realistic cloud formations and night time fireworks getting reflected on car models. No self reflections on cars either