The reading level filter was designed to enable searchers to filter out content by basic, intermediate or advanced reading level material. This was a nice feature for experts on specific subject matters to filter out the basic stuff, while newcomers to a topic to be able to zone in to the more basic content.
Google was very proud of this feature, claiming they hired teachers to evaluate tons of content through the web to fine tune this filter. I guess, like the many other search filters Google has dropped over the years, that this filter didn’t get as much use as Google would have liked and Google decided to remove it.
Here are pictures before and after from Alex Chitu: