All the Classic Companions exits are sudden (With the exception of Teagan harping all the time about being over it). In the case of Jo Grant though, at least there was a 6 episode story where she fell in love and got engaged to give credibility to her hasty exit. Liz Shaw simply stopped showing up to the lab, and Jo showed up in her place, so, Liz didn't even get an exit.
While Tegan did spend much of her early time on the TARDIS harping about how she wanted to leave, she seemed to tone it down a bit when she rejoined the Doctor & Nyssa in "Arc of Infinity." So when she decides that she's had enough of seeing good people die at the end of "Resurrection of the Daleks," it felt pretty sudden to me.
But at least there was some chemistry between Jo & that hippie scientist in "The Green Death." When Leela seemed to fall in love with some random Time Lord security guard in "The Invasion of Time," that was the most outta-left-field departure of any companion that ever got an on screen departure. (I think they should have done some last minute re-casting & rewriting so that the Time Lord that Leela falls in love with is the communications officer that accompanies Leela when she leaves the dome. At least she seemed to spend some significant amount of time with that character.)
I kinda like what they did with Liz though. Granted, it's a shame we don't actually get to see her say goodbye. But I like how the Brigadier seems to frame Liz's attitude at the time she left. "Nonsense. What you need, Doctor, as Miss Shaw herself so often remarked, is someone to pass you your test tubes, and to tell you how brilliant you are. Miss Grant will fulfil that function admirably."
I also like how the Brigadier is obviously trying to pawn Jo off on the Doctor so that he doesn't have to deal with her too much personally.