Yes you can first use the profile endpoint https://api.chess.com/pub/player/sjbfan and get the "joined" timestamp. Then at least you'll know which accounts existed when. But that doesn't solve the problem that some people may have created an account but may not have archives for specific months because they haven't played.
The best way to solve this would just be to get the available archives first for each username, then you know for sure they exist and then download them. https://api.chess.com/pub/player/{username}/games/archives
I'm trying to build backward on to a series of databases that start at 2021, that someone else has put together. One PGN database for each month, approx. 3-4GB each, starting at 1950 elo. I was able to query for all the names of titled players, and along with just brute-forcing the copy and paste thing with the leaderboard, gathered about 21,000 names that stretch back to 2300 elo. (Since my own project using that series of big PGNs already filters starting at 2300, that works just fine.) I built query URLs for each name and downloaded them one-at-a-time in a download manager. Only half of these URLs returned PGNs for 2020-12 and 2020-11, and I expect the problem will only get worse as I go further back in time.
Is there any way to find out who used to be a member here? I realize that many of the names would belong to banned players, but also just people who've decided to go elsewhere. It would be pretty hard to do it manually, and I can't think of how to do that anyway.