If candles can go on cakes, torches, lanterns and candles can go on bottom-slabs.

With 1.17, a feature was added that allowed candles to be placed on cakes. With the addition of this feature, it sets a precedent for smaller placeable items to be allowed to be placed on bottom-slabs. Now I wouldn't expect fences or walls, but it make perfect sense for candles, torches and lanterns to be placed on bottom-slabs, similar to the Torch Slabs Mod. I feel this is a very vanilla-feeling feature, and should be in the base game.