I mean writing things I'd rather not have on my permanent record. E.g. an overly emotional and poorly thought out political manifesto, or giving personal details that people I work with might consider 'too much information' or "overshare"

Especially for the latter, a pseudonym seems polite. I mean, you need to be aware that if someone /really/ wants to find out, they will. But using a pseudonym, I think, is a polite way to separate your personal life from your professional life.