I feel like the best “sitcom” I’m watching where the characters don’t change too much is probably Bob’s Burgers. However, the characters have kind of evolved over the few years it’s been running.
the ‘setting for jokes’ style sitcom without any need for internal growth still exists, but sitcoms which focus on growing and exploring characters within the format are definitely more common now. just off the top of my head, Bojack Horseman, Unbreakable Kimmy Schmidt, and Santa Clarita Diet all allow their characters to change and flourish in different ways over time, sometimes in pretty dramatic ways, and I think you can give Community a (small!) portion of credit for that trend.
one of the things i really like about Always Sunny In Philadelphia, incidentally - taking into account all the issues with that show - is that they set out explicitly to do the opposite, to make the characters ever more cartoonish and irredeemable, to push them to increasingly depraved depths and to have them never learn anything. it’s funny, because the characters do ‘grow’, but not in the way we expect; they grow worse, and they gradually destroy and corrupt everything and everyone around them. it’s a striking choice, to me, to have a show with no ‘straight’ character in the main cast, where nobody ever learns their lesson, and where the only progress is further into the toilet.
There also Scrubs, which is pretty old now I guess, where the main characters evolved. The whole show is really about JD refusing to mature even when put into situations that should force him to, and then finally embracing it at the end of the show. Parks and Rec did this too to an extent, if i remember correctly. I don’t think this type of sitcom has been common though.
I don’t think the genre has evolved, but as others have said, it maybe is becoming more common to have this type of writing in sitcoms.
And then they brought him back for season nine for no reason after giving him one of the most beautiful send offs a character could ever have and he instantly regressed and had to relearn everything before he left again and we didn’t get enough time to really cement the new characters because we were so focused on the old and then a promising new idea for a sequel series died after one season.
Yes I am still bitter about this.