I write YA fantasy, and ... I have the blood of a lot of mentor characters on my hand.
I mean, I know it's trope, but the reason mentors (be they parents, guardians, teachers) die in YA novels is pretty obvious: good parental figures DON'T let their charges get into the sorts of danger that is required for an exciting story to happen, and if you write a story where the character's dad keeps stepping in to save him, it would get dull really fast (or the main character would switch to the dad because he's the one doing all the real heavy lifting while the kid just kind of sits back and goes, "Thanks, Dad.").
I mean, there are OTHER ways to sideline those protective authority figures so they don't get in the way of a good story.
1. Portal fantasies provide a natural separation where the teen protagonist can't get in touch with his parental figure because of magical distance.
2. The parent can be the one IN danger forcing the kid to go rescue the parents (kidnapped parents are a pretty big trope too).
3. The parents can be awful and just not care if the kid gets into danger.
4. Boarding school/Academy situations (but then you get into the "why are all the teachers so awful?" thing that people complain a lot about in Harry Potter jokes).
5. Illness/injury can sideline and adult for long enough for the teen to step out of their shadow.
But the mentor death is popular for a reason because it also provides (generally) a motivation to move the character forward (bad guy killed my mom. Now it's personal) and shows that we're in serious territory now.
And I have written a few mentors who have made it through the book/series alive (Sylvis from the Green Princess, but hers was mainly because her student surpassed her in ability by a great deal .... plus Kit was like 20 at the end of the series so not exactly a little kid who needed protection) ...
So when you start a YA novel do you expect the protagonist's mentor to die? Are you relieved when they don't? Do you have a favorite example of how a mentor figure managed to stick around without the MC taking a backseat or the mentor looking like a jerk for letting a teenager do all the dangerous stuff while they sat back and spouted sage advice?