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".
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Where are you seeing full-text search for groups? I can filter the post titles on the current page of listings, but this is completely useles...
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".