The last Impala ragtop was '72, many were highly optioned to Caprice standards. '73 to '75 they made the Caprice Classic convertible, but it used an Impala vinyl interior with Caprice options available. They made no convertible for '76, and they also stopped making the pillarless hardtop coupes (like my car, that roof was only used in '74-'75 and only on Chevys) where available...they did make the B-O-P coupes in '76 that you could technically consider a hardtop because the skinny Colonnade quarter window did roll down.
As for Cadillac, they kept making the Eldorado through 1976. After that no factory ones were made, but some coachbuilders did, as mentioned.