[Note, this was on a wiki using a somewhat old version of software]
I had a test account I wanted to delete. I went to user merge and delete, selected the delete checkbox and left target blank. I selected ok to merge to Anonoymous.
Now in RC there is:
> (User creation log) . . User account <IP address here> (Talk | block) was created by Wiki Sysop (Talk | contribs | block)
This is kind of surprising behavior to me.