I’m going in May and since DH is sure to order it, I can certainly ask if it’s available and report back but I’m willing to bet someone will make it there before I do and hopefully can fill us in.
You can still make special requests if you mobile order. When you get to the restaurant you still check out with a cashier and can make modifications then.
I did find a 2017 review that said they asked and horseradish was not available and they wished they brought Arby’s horsey sauce from home. If not having horseradish is a deal-breaker it might help to see if there’s another menu item DS may want