A counterpount: You can be a young user and not realize yet how wide is public sharing, you may be in an altered state (griefing, drunk) or be pushed around by disability (adhd, or other cognitive impairements). IMO these are not all qualifying as “being a dumbass”.
Additionnally, it’s possible nobody is able to correctly predict the future. Are your thoughts today going to cause you grief tommorrow, in a year, in 10? How about you post today about how good the orange juice you had was, but that is later on an excuse to put you in a labor camp with others due to the fact that eating fruits is now seen as unholy and that you are a sinner, in need of correction?
There is a balance to be struck between “all is public and it is your own fault”, and privacy nightmare. There should be some teaching happening. As somebody else mentions, having a habbit to use Matrix for DMs and more personal matters is one of these good habbits.