Take Reddit. Besides the well-known archive sites that keep removed/deleted content (removeddit, ceddit, etc), there is this cool service (pushshift) that offers all that and more. One interesting and relevant feature of this tool is the ability to look up all content by a specific author/username. You just put the name there, and lo and behold, everything this person ever posted is right there; even if it's deleted, or removed.
If you do a quick test using this tool, you'll soon see the amount of people who thought their deleted content couldn't be tied back to them, continuing to use their usernames like nothing happened. The deleted content can sometimes be incredibly sensitive information that the person posted by mistake (or when looking for advice, having a rant about something, etc). It sometimes includes very specific details that may uniquely identify a person. And most users aren't aware of any of this. And it's available even if you delete your account. Someone just has to know that you used this username once upon a time.
The tool eventually removed this option from the UI (but it's still available through their API). The thread where this removal was announced is interesting and reveals how useful this tool was to some users on Reddit for combating spam/harassment (or even to find their own content), but nevertheless this dark side of it is there. As you might expect, someone slapped a new UI on that very tool, and that feature is readily available today, few google searches away.
When I saw this tool, it really reminded me of that. It's incredible how our online activities became permanent, no matter how much we try to erase the parts we don't like (or don't want to be public).