In-service Roth rollovers--protected benefit?

Plan allows for in-plan Roth rollovers at any time as long as the funds are 100% vested. 

Is this a protected benefit?

Don't think so.

But if you had in the plan, allowed an owner or HCE to take advantage of, and then amended out - you might have a discrimination issue, especially if the provision was not in the plan for a while, or was amended out before NHCEs became fully vested and were able to utilize.

This IRS website page (blah blah blah not official guidance disclaimer dot dot dot) confirms it:

https://www.irs.gov/retirement-plans/deadline-extended-to-add-new-in-plan-roth-rollover-provisions