I had seen a clever solution to an excel problem online which brought me to Stack Exchange's "Super Users" service. But I have forgotten the user's name as well as the excel problem that I had researched on the internet, so I cannot find that user anymore. The only information I have is that the user had not been active in Super User for a couple of years, plus I remember the profile picture. Is there a list of users I can bring up in here who have not been active since 2021, so at least I could narrow my visual search?
This looks to be a Problem X Y. You want to find a questions, but instead of asking for help to find the question you are asking to find a user. I think that that you should focus on searching based on the content rather than the user's avatar as it has been a couple of years inactive and could have changed at any time.
Superuser hasn't a list of users based on their last active date. The page of users is the following: Users - Superuser. It has tabs to view users by Reputation, New Users, Votes, Edits, and Moderators. At the bottom of the page, you can find the links to the Reputation Leagues page.
On StackExchange you can find the Stack Exchange Data Explorer. From it you can create a query to list users based on their last active date among many other many things. It has help page. Also, you can learn about it in Meta Stack Exchange.