I care for 6 OTTBs, and ride a few others in addition to those.
We don't feed grain. These are all polo and ranch horses (meaning they work) and we don't feed grain. Granted, we don't want our horse overweight, that would be cruel considering they are asked to do a lot, but we also keep them very healthy.
Before my current position, I used to get OTTBs and re-work for riding mounts. I didn't grain them. Worked on tracks in the eastern states for several years and no one I knew fed sweet feeds.
Also, we ride them when we get them. If they have been on the track, they can stand, walk, trot, and canter. Unless they went "rogue" on someone or are lame, I see no reason to put off riding.
We do feed a senior feed to two semi-retirees TBs that are in their mid to late 20s that small children ride.
So, once again there are many ways to treat a horse, and they mostly all work!