As you can see on http://www.geocities.com/otelloverdi/luanne.htm , but as you undoubtedly knew already, Luanne appears in fewer and fewer episodes every year. (This season she's so far been in only three new episodes, plus one leftover from season 6 and another leftover from season 5.) But it's interesting to note that Cotton Hill, who doesn't have a super-busy voice actor and who now lives in Arlen rather than Houston, is also appearing less and less. This season he's had one big episode ("An Officer and a Gentle Boy"), a one-line cameo in "Full Metal Dust Jacket," and that's it, except for a couple of wordless appearances.
It seems to me that something the show has been lacking lately is an ability to really use its community of supporting characters. Cotton is a good example, because in the early years they found all sorts of ways to use him even though he didn't live in Arlen: He appeared in flashbacks in the first three episodes; turned up in a dream sequence in "Hank's Unmentionable Problem," arrived at the end of "Movin' On Up" to lay the smackdown on Luanne's annoying housemate, etc. But now the writers don't seem to write for him unless he's the focus of the story. Same with Luanne: If the story's not about her, they don't seem to find it worthwhile to drag Brittany M. into the studio (whereas even as recently as season 5 you would sometimes see her making appearances in episodes that were not about her, i.e. the Itchy Algae episode: "I'm sorry I can't help you, Bobby, I don't use newspapers"). And it seems to me, though I haven't checked this, that supporting characters like Connie or Kahn or Mr. Strickland appear less often than they used to.
I think this lack of the supporting cast makes the show feel somehow less rich than it used to. Take "Ho Yeah!" which was billed as the 100th episode (the official 150th is coming up soon). That episode really isn't about anyone except Hank, Peggy and the guest star; yet there's room for lines from Luanne, Bobby, Cotton, Mr. Strickland, Mike Soto and Lane Pratley in addition to Hank's buddies. Now a lot of episodes seem to be basically just Hank/Peggy/Bobby + guest characters + a few token lines from Hank's buddies (and Dale has gotten so idiotic that he's close to Homer Simpson at this point). I know that Hank and Bobby's relationship forms the core of the show, but the show has built up a large community of characters and a realistic world in Arlen -- the episodes would feel more "complete" if they'd use that world more fully. Starting with giving Luanne, Cotton, etc. some scenes in episodes that aren't about them (remember Luanne's scene with Bobby in "Dang Ol' Love?" that was a great scene, and it was only last year).