To give a brief history, yes these road widening setbacks (see section 53(1) of Land Use Bylaw 1P2007) date back to the 60s I think. They were intended to add lanes of traffic. So, 5.182m on both sides of Edmonton Trail was intended to add a lane in each direction, making it a 6 lane road. Luckily, not all of these happened (16th Ave through Montgomery is 10.363 on the south side, and the intention is to allow for a 6 lane road eventually and it is still on the books I think).
Since then, the table has lumbered on in existence, without any major reforms unless a major road widenign project was undertaken (16th AVe from 14th West to 6th East for instance). Roads continues to demand it be protected, from heaven to hell, no matter the circumstances. Downtown finally received some compromise, allowing developers to encroach in this space starting at the P2 level of the parkade, and 9.0m above the grade level (so 3rd floor could cantilever over). Any non-city centre development seeking the same relaxation was typically stonewalled. This is why nothing has been built on the north side of 16th Ave since the ARP was passed over a decade ago. Losing 5.182m of your property meant any sort of parkade was just not feasible.
Fast forward, and the new CTP and Complete Streets policies get approved, essentially confirming that we aren't going to widen all roads all the time. However, as seems to be the norm, not all policies/bylaws were updated to reflect these new overarching desires, and the stupid bylawed setback table continued to lumber along. The one positive is, the table protected for wider boulevards, allowing proper room for things like boulevard trees, wider sidewalks and cycling infrastructure. However, this still doesn't solve the problem of inefficient parkades, and developers even offering to build a sidewalk on their parkade, and set the building back with a public access easement, was typically met with a refusal.
Currently, there is a consortium of inner-city developers meeting with City representatives on how to improve the process for redevelopment, making things easier/cutting red tape. I have heard this table is high on their agenda, but not sure what progress has been made on it.
