> The idea was that you could browse a map, and see all the ones along the route you were thinking of traveling, and decide which spots were worth stopping at.

Strava has something like this, not specifically for informational boards/plaques. People with public profiles can have their uploaded photos of (usually) trail markers, plaques, rickety bridges/stairs/rocks, big trees etc added to a community map so others can see whats on a trail or route they want to walk/run.

I take the exact photos you are talking about and have them uploaded to my trail activities -- though I keep my profile private, more of a historical record for myself. I've wondered if others were into taking/saving photos of all the ones they come across. I see others reading and moving on or taking a selfie, but am usually the only one trying to get photos of the board/plaque and the objects it was pointing out