This morning’s Chicago Tribune had an article stating that this year they know of a dozen students who have had their admissions revolked over something they posted on social media this year alone.
But having said that, an admissions officer spends about 8-10 minutes looking at your application - total. That doesn’t leave a lot of time for internet searches about you, much less examining your cookies or social media posts. For them to withdraw your offer of admission, someone pretty much has to bring the incident to their attention.
Short of a major much-publicized felony, it is highly unlikely they will care what you did in middle school.