Depends on how big of a slider/trailer and your turnout situation along with if he's the type of horse that can be responsible for himself turned out. If you take a horse that's never been turned, turn him out on 50 acres in the rocks in the brush, he's going to hurt himself whether he's got sliders or not.
We used to duct tape sliders for turn out. It helps from them hanging the trailer on anything to pull it off and it gave a bit of traction in a turnout.
As far as riding in them outside, again depends on how big of a slider/trailer and where youre riding. Ive ridden them outside on frozen ground gathering cattle but Im careful. Its good way to stifle or split their hips if they get fast or slip. But that is with a cow horse slider not a big reiner one.
This makes me sad. It sounds like he got asked to stop too hard and/or had too big of a slider on, slid and he got scared. Regardless of breeding most horses if physically capable, will take a sliding stop if introduced to it correctly. Once you scare them especially as a youngster, it's pretty hard to get them over it. Don't get me wrong some horses love it and some will just do it but they shouldn't be terrified of it if it was taught correctly.