Waypoint 21 D marks the north intersection with a loop trail which heads west to Haanel Drive and back. The sign here labels the next waypoint as 20 E. I suspect that the next waypoint is actually 21 E. Interestingly, at that next waypoint, the post is still in place but in June 2019 only the map is on the post.
The National Capital Commission (NCC) maintains waypoint signs throughout the Greenbelt, a natural area which surrounds the city of Ottawa. The waypoints mark trail intersections. The signs include geographic co-ordinates and a map of other waypoints.