In practical terms:
- If you remember the user's display name, Google search
username unregistered site:math.stackexchange.com/users
will find the user. Usage example.
- If you interacted with the user in comments, there are ways to search for those comments, as described here.
By the way, I do not see any reason for SE to hide unregistered users from the list. They are recorded in the Users
table just like
registered users. They have publicly visible profiles, which are indexed by Google. Their information is present in the Data Explorer database, and is also
available through the Stack Exchange API. And many of them are/were more active than a median registered user. The median score of
a registered user is 25; about 20% of unregistered users have at least as much reputation.
The unregistered users with reputation 500 or above (as of December 28) are listed below.
- bof 3903
- Katlus 2037
- M. K. 1124
- Alfred Chern 922
- maurice 891
- binn 861
- Matt 848
- nikkita 796
- opt 750
- admchrch 713
- fosho 702
- George 696
- anonymous 659
- anon 646
- Carlos Ribeiro 631
- user70520 624
- Kerry 619
- El Moro 603
- Jj- 600
- ray 597
- DBFdalwayse 595
- Amzoti 590
- Cody 543
- xan 504
I also made a complete list of unregistered users
sorted by reputation (warning: large file, over 3MB). There were 22410 unregistered users at the time of data harvest. Their median reputation is 11.
If some now-registered user recognizes their name on this list, they can request a merge.