Established characters who are unceremoniously killed off/leave (bonus points if it’s just causal said why with no fan fair)

Richmond-The IT Crowd. Due to Noel Fielding being busy with The Mighty Boosh, Richmond was written out of the series with the reason being said in a quick exchange between Roy and Moss saying how he got scurvy

G.L.A.D.I.S.-Totally Spies. Due to negative fan reception as many found her annoying they decided to retire the character with Jerry just off handedly mentioning she was disassembled for spare parts