Should be locked to "Normal" difficulty. Item levels will be lowered (set like Timewalking dungeons) to a cap so there won't be unrealistic requirements in the LFG tool (except that rare tard that expects people to have a legendary in every slot).
LFR would just be a train wreck like the Molten Core anniversary was. If it was normal you could easily prune out problem causers without relying on a vote kick system which hardly works and has limitations.
Or at the very least have LFR and offer Normal mode for people that don't want to have a horrible experience.