Basically he has been doing other things for years but it looks like the producers wanted to slap an exclusivity clause into his contract.

So this effects characters like Flanders, Reverend Lovejoy and many more.

Between this and the loss of Marcia Wallace who voiced Edna Crabapple, the producers and voice actors need to start seriously considering that it is time to wrap it up.

That being said, Simpsons is returning for Season 27 and 28

However they need to consider what would happen if for example they lost the actor that plays Homer among other voices or if they lose the actress that does Bart.

Sure you can get soundalikes but the change would be noticeable and likely not well received.