Harry Shearer the voice of Mr. Burns, Ned Flanders, Principal Skinner, among others tweeted he received a letter kicking he off the show for pursuing other opportunities outside of the show.
According to tweets sent by Mr. Shearer, The lawyer of "Simpsons" producer James L. Brooks stated 'show will go on, Harry will not be part of it, wish him the best’ Shearer goes on to tweet " This because I wanted what we've always had: the freedom to do other work."
No one is really clear as of yet what, if any, of these outside activities have led to his departure from the show. Per Entertainment Weekly sources have said "...the actor would not agree to record his dialogue 'in a timely manner,' unlike the rest of the cast."
There is hope he the issues may be resolved and he returns to the show. If not the show will go on; but it will never be the same.