The conclusion of a piece should be the wrap up, or the judged reasoning of what you have come to from your analysis.
In a scientific article, the conclusion lists what the end result of the experiment was; Was the hypothesis correct? In terms of an English essay, it's similar. You don't introduce anything new - a physicist won't suddenly add new information in the conclusion, "Oh, by the way, I did this test three more times with this different liquid".
In your case, it varies. If you're introducing evidence, for example, a quote that states that there is more income equality or lack of social mobility, or discussing how the text relates to modern day issues, that is considered adding new information. However, if you are using this as our overall conclusion, for example by stating:
From [author]'s use of [techniques], it is evident that the author uses
class as a signification of the lack of modern social mobility, and
the higher income inequality in our world today.
In order for this to work, however, the entire piece should be written in a way that means that this conclusion can be reached. All points that you make in the essay should relate back to the idea that there is higher income inequality, and the link between the book and real life should be stated early on. For example, by stating:
The author's use of [technique] in [quote] can also be attributed to be a social commentary on our modern increasing amounts income inequality that has become prevalent in modern life.
So - ironically - in conclusion, the conclusion should be a wrap up of the entire piece that summarizes information stated before hand. It varies based on what information you're intending to use in the conclusion, however as a rule of thumb - you can always introduce the concepts early on, and carry them along as a theme for your entire piece.