10 Best Doctor Who Companion Departures (And 5 Worst) from SFX.
Some great examples to follow, and some... less so.
DWAITAS discusses the end of a character’s run, giving it more thought than many RPGs, reflecting its importance (sometimes properly addressed, sometimes not) in the series.
The Bad Trait “Unadventurous” in its Major form is expressly designed to write characters out, which may be a bit much but certainly reflects some of these departures.
Otherwise, the general advice is to make a departing PC’s (and particularly a departing player’s) last adventure as big a deal as you can (depending on how much warning you get, of course) and that’s advice worth taking to other games.