The System account is not a normal user account. No one can log into it. It is part of the Discourse software.
A lot of early poster’s names were lost during a software upgrade many years ago. IIRC under vBulletin these posts simply did not have a user name associated with them. Since there was no user associated with these posts, Discourse just displays the posts as belonging to the System account, like it does with any other post that no longer has a valid user associated with it.
The 2020 example was a spammer. That post should have been in the cornfield (and now is).
The System account also does a lot of other things, like automatically silencing spammers, for example. But if you see at as a post author, it means that the real post author’s account no longer exists.
If you quote the entirety of the post directly before yours, Discourse deletes that quote from your post (which some folks complain about, but that’s a topic for a different thread). Automatic post edits like this also show up as being done by System.
System is a busy guy for someone who doesn’t really exist.
I have no idea what happened to Collounsbury.