You can’t throw a stick without hitting someone that has posted a youtube video about making a router sled and using a router to plane a slab of wood flat. They have become very common. I have covered making a router planing sled and what router bits to use for planing. Usually router sleds are used to handle materials that are too wide or too thick to feed into a planer. A router sled excels at planing gigantic slabs so it makes sense that people would feature them that way. However, a router sled plane can also do something that I have not seen anyone address, create surfaces that are not parallel.
For visual appeal on a coffee table that I am building, I wanted the slab legs to taper from wide at that bottom, to narrower at the top. This was pretty easy to achieve with a router sled, and rails that could be set so that one rail was lower than the other side.
In this setup you can see that the rail on the left is an inch lower than the rail on the right. Notice the piece of scrap wood on the right to keep the slabs fixed in location.
I started planing on the left, which is to be the thinnest portion of the slab, moving toward the thickest section on the right. Keep lowering the bit and repeating passes until the surface has been fully removed all the way across. Then the taper will be complete.
This simple change in method results in tapered slabs, which give a little bit more visual flair to slabs. It gives them unexpected angles that can sharpen a piece, or soften it. Not everything has to be co-planar. Experiment. Have fun.