Some other artists probably have. If archival releases count then you always have your greatful deads dropping 60 CD box sets which they’ve done i think 3 or 4 times now (they just released another one one of these a few months ago). As for new studio material, the recently discussed King Gizzard released 5 albums and one EP in 2022, two of which were double-length. That might not quite hit Zappa’s numbers in 79, but also a lot of the Zappa material from that era was live material and holdovers from Lather which was intended to be released in 77. So it could be disputed how prolific he actually was in that year.