The old bridge and road were closed back around '79 or '80 when the current one opened. I didn't start biking down there until '90 or '91 and it's probably been 5 years since I've last been down there. But, when I last rode from 77 to 35W on the two-track and singletrack next to the river, I don't believe there were any signs that indicated it was questionable activity. There were refuge signs facing sideways along what I assume was the border of the regulated area. There also may have been a couple no unauthorized vehicles signs that were the same as the ones at both ends of the gravel road back into the singletrack west of 35W and I assumed those referred to cars. At that time the trail through that section was well defined though it was less so and more raw than the bluff side or the stretch from 35 to BFR. I vaguely remember a gate in there somewhere but there was trail going around/passed it just as there is (or was) heading east from the boat landing so, again, I assumed it was just to keep out offroaders looking for a mud bog.
I guess, if a ranger said no, that's the official story.