You've got a user interface problem there, not a data one. The Monarch is fully credited on IMDb. Here he is in the credits for
Episode 3 of season 1.
The problem animated television shows particularly create for IMDb is that frequently one person voices many characters. IMDb (and particularly the Full Cast and Crew page you are usually viewing) is primarily focused on telling you who the REAL people involved with a production is and only secondarily on the actual content of the productions.
So, Christopher McCulloch has voiced literally a couple dozen different characters on that show (on this IMDb page you can see
them all), so when you look at the cast list for an entire show it is focused on showing the cast, not the characters. Go to the actor page, or the individual episode pages and you get the detailed information you were looking for.
It isn't at all clear in the UI, but when you see an actor listed and his character is shown as Bob Smith / ... the "/ ..." part indicates that there are other characters not listed on the page. When a character plays just a few roles (such as Eddie Murphy in
The Nutty Professor) they all show but they seem to have a cutoff when it is more than a half-dozen or so.
Thus, Harry Shearer, at first glance is just the voice of Ned Flanders on
The Simpsons. But go
here and see every voice he has provided in 413 episodes.
It is definitely a poor piece of UI design (at minimum the ellipsis should link to the detailed section of the actors page. But it isn't a new element of their design so isn't a sign of any recent decline.