I would say that is not best practice to use URL for SMS opt-outs, even almost everyone have a smartphone there are a lot of people with feature phone. If you already have SMS channel, consider to create web service for SMS opt-outs (ex. customers sends STOP on short number 1111).
It's mind-boggling how no one in the link-shortening industry's been able to spend the 10 minutes of engineering to implement url param fwding. To solve this can make your own shortener that fwds params (and put all the other shorteners out of biz), or write js in a workfow activity or control typology to shorten url's as they pass through.