Will flat earth society membership affect my graduate school application? Just to be clear: no, I don't believe in the flat earth theory.

However, a few years ago, I thought that the flat earth society forum is a good place to practice my debating skills and to have fun. Therefore, I joined it and made an online id that's close to my real name to debate on the forum. It just made me feel accomplished when I see people having trouble disproving my obviously faulty "theory". I posted a lot there in the past year.

I plan to apply to a master program in physics and I worry that people will find out my membership, think that I believe in pseudoscience, and reject my applications. Is this an issue I should address on SOP?
No, I do not think an admissions committee would check for this type of information.  Nor would they assume the poster with the same name is the applicant.