That is true. Originally, Lady Sybil (beautifully played by Jessica Brown Findlay) had signed up for the first three series' of Downton.
Needless to say, at the time, nobody knew then that it would take off and become as enormously popular as it did, and when that happened, Jessica Brown Findlay made it clear at the end of the second series that she didn't expect to be around after the conclusion of the third season, which, in effect, gave everyone a year's notice.
So, her departure had been discreetly signalled in advance. Thus, Julian Fellowes was able to write her out of the series in a natural way (and that was a superb episode, genuinely harrowing and shocking).
I agree about 'an actor' "seeing stars". His departure was a lot more abrupt, and had not been signalled clearly. Moreover, apparently, he had been offered the option of appearing briefly, or for one or two episodes in Season Four, or even being killed off in the first episode of Season Four - all of which he turned down.
Inexplicably, (and I know I am not even responding even remotely rationally to this - he is a professional actor, after all) but I have not watched him in anything else subsequently.
While he has a perfect right to promote his career, I do think, however, that his exit could have been more graceful. Although he was criticised at the time. this was not in any way the fault of Julian Fellowes, who dealt with an abrupt departure as best as he could.
The smart actors keep their backup gig going while gingerly testing the waters before making rash changes.