Everyone on any part of the internet should be very wary of considering any of it private, especially somewhere as big as MN.
The way to privacy is to name change regularly, but even with that you need to be careful.
On a different forum I once recognised my aunt's friend's daughter who I had never met, because she posted about something innocuous that happened at her wedding. After that, I knew she was TTC before her mother did, among other things.
And I've posted that anecdote under several usernames over the years, so it can now link all of my posts.
If you're going to post about something that happened at school or work, you'd better name change or someone will recognise you and then know how you really feel about your MIL, the struggle your DC had with biting as a toddler and that you find smear tests very uncomfortable.
Deleting posts is no good, there may be screenshots or old posts can be accessed in other ways.
Just as we're forever telling our DC, once it's on the internet it's out that forever, and a MN username is rarely as anonymous as we think.