I've got some that come to mind:

• George & Leo (1997-98 Bob Newhart and Judd Hirsch sitcom because I would watch those two sit in a white room together and just stare at each other)
• Coming of Age (1988-89 sitcom where Paul Dooley plays a freshly retired man who moves into a retirement community kicking and screaming. Two of his neighbors are the always wonderful Alan Young and Glynis Johns, and Bob & Ray memorably play two other annoying neighbors in one episode in their last time appearing together as a duo)
• Mary (1985-86 Mary Tyler Moore ensemble sitcom that's pretty similar in style to her legendary 70s sitcom but she's a newspaper columnist. John Astin and Katey Sagal play supporting roles!! Katey is especially wonderful on this, she plays a cigarette smoking cynical advice columnist and the best episode of the entire show ("Same Old Song") is all about her character and really shows off her acting chops.)
• Carter Country (1977-79 sitcom that you could say was a rural and much less clever version of Barney Miller but it's got a lot of charm with Kene Holliday and Victor French being especially likable)
• Bob (Bob Newhart sitcom from 1992-93 that was as charmingly fun as anything else he's ever done and deserved a longer run, though I think the format change did way more damage than they were ever anticipating, and that's even with them adding Betty White to the cast. They even did a real life shortlived Marvel comic about the character he created on the show!!)

There's a ton of others (I recently got to see an episode of Calucci's Department for the first time and couldn't believe how surprisingly wholesome the plus-sized romance storyline was in the episode) but I think I will stop there.

Of "underrated" sitcoms that WEREN'T shortlived that I feel like people tend to forget about now-a-days, I really love Amen. Secretly I might even like it more than The Jeffersons if we're talking Sherman Hemsley sitcoms.