By excluding the big providers, the searcher is trying to filter out the noise of public forum posts and generic discussions. They are hunting for corporate domains ( @deloitte.com , @bankofamerica.com , etc.) that appear in raw text files alongside the year 2021.
-gmail.com -yahoo.com -hotmail.com -aol.com filetype:txt 2021 site:.gov -gmail.com -yahoo.com -hotmail.com -aol.com txt 2021