I had tried this site a year or two ago and found it unusable then, but it seems greatly improved now. I found posts as old as 1982, but recent coverage seems to stop around April 2022. Crucially, it supports full-text search on posts within a specific group - something which my own site https://newsgrouper.org cannot do. I find the user interface a little awkward, but it does now appear to be a really useful resource.
As I said, the user interface is awkward. You need to select the "Content" checkbox when searching for Posts. E.g. to search for "deadlock" within comp.lang.tcl, start with https://usenetarchives.com/index.php?s=deadlock%20ingroup:co...
then select "Content", unselect "Author" and "Subject" and click "Search".
