Minor Peace Talks spoilers to follow.

Isn't it annoying when Harry is trying to figure something out, but a mentor figure has a vague reason why they can't tell him?

Examples:

In Turn Coat, (in Ebeneezer's journal entry), Rashid tells Ebeneezer not to tell Harry about Demonreach, because it won't make a difference?

PEACE TALKS EXAMPLE:

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In Peace Talks, River Shoulders say that he's not the right person/it's not the right time to tell Harry about wizards who are hanging on, or about starborn things.

Ebeneezer suddenly decides in Peace Talks that it's "time to start teaching you about this starborn business."
to which Harry responds, "You're going to start talking about it now?" As if Harry has tried to ask him about it before but got stonewalled.
Then, after the cornerhound fight, Harry presses Eb for more info.
Eb responds, "I already said more'n I should've."

What are other examples of this happening in the series?
I'm sure there's some with Uriel, but the reasons that he can't tell Harry things are probably more cosmic limitationsy than what is restricting others.

I understand the Doylist reason why people don't just tell Harry stuff about being a starborn/demonreach: narrative tension, mystery, etc.

But what could be the in-universe reason for why people won't tell him things? What is keeping these mentor figures from telling? Why is it a bad idea?

In extension, what is the right time and person to tell Harry these things? Is there some kind of starborn tutorial that he'll get in the future, and they don't want to spoil it? 

Some ideas for why this could be happening:

FORESIGHT:
It reminds me to the Harry/Bob conversation in Proven Guilty, where the Gatekeeper has given Harry a vague note. Bob explains that it is vague likely so it doesn't mess too much with the timestream, as the Gatekeeper got the knowledge from the future.
If the people like Ebeneezer, River Shoulders, and the Gatekeeper got their knowledge from the future, this could work. However, that's sketchy because that's a lot of specific knowledge that a lot of people have.

ANTI-SELF-FULFILLING PROPHECY:
A self fulfilling prophecy is when you hearing the prophecy causes the prophecy. (You hear a prophecy that your son will kill you so you lock up your son, thus making him angry enough to kill you, which he does.)
Perhaps the mentors are worried that if Harry knows about the starborn prophecy, he'll just end up messing it up and not fulfilling it.
For example, it gives him motivation to turn evil? It will scare him into giving up before he even starts if he knows how hard it will be?

STRICT NEED-TO-KNOW CONSPIRACY:
The secrets of starbornness are protected by a conspiracy? The more people who know about it, the more risk it is. So, it's very strictly need-to-know.
That's hard to believe, because wouldn't being Starborn be the definition of someone who needs to know about being a starborn?
This could work if they are unsure if he's really a starborn/the warden of Demonreach, etc. The evidence points that way, but they are still waiting for it to be 100% confirmed before they tell them something.
However, that's hard to believe because it would make Ebeneezer's actions in Peace Talks very confusing:
The conspiracy could make a little more sense if the people brought into it were required to take a magic oath of secrecy. Like they literally can't tell Harry any more, even if they want to and think he should know (similar to the restrictions we see on Lily/Fix in Proven Guilty).

What are your ideas? Why won't people tell Harry things?
And what is the thing that they won't tell him?

[This is a Peace Talks spoilers thread, proceed at your own risk ]

Rashid = Starborn. There's been a bunch of great threads talking about this idea, but I wanted to re-open it with the new info we got from Peace Talks.
What do you guys think?

So, there's a WOJ that Rashid killed Mad Arab, Abdul Alhazred. And he died ~738 AD. This puts Rashid's age at 1300+ (assuming no time travel shenanigans there).
So, 666 + 666 = 1332. That puts Rashid in a probable Starborn birth time.

If the big Starborn advantage is resisting dirt when dealing with outsiders, that would make him a perfect candidate for being the Gatekeeper. If you had someone in that position that got tainted by the outsiders, it would turn into a reality-ending liability real quick. So, they couldn't accept any non-Starborn applicants for that job (unless they were immune in some other way).

WOJ also that "Rashid was Harry last time around, and didn't enjoy it much." It clicks that this means that Rashid was the Fulcrum, the Starborn, the person who was in the right place at the right time.

River Shoulders offers to mentor Harry in Peace talks.
Then, he says,

“Lot of the wizards who matter are near the end. Hanging on hard.”
I tilted my head at him. “Why?”
“Not the right person, time, or place to tell you, starborn.”
I pursed my lips. “Six hundred and sixty-six years,” I said experimentally.
River’s craggy brows rose, itself a feat of superhuman strength. “Huh,” he said. “You learned some things.”
“I learned that,” I said.
“We pretty close to that time,” he said.

I would interpret this as, "Our current Starborn is about to die of old age so it's pretty close to that time when you need to be well trained to pick up the baton."
Rashid has been hanging on hard, through Nevernever time dilations (and who knows what else). He's got to be close to his end.

But what about the Starborn that was born 666 years ago?
Perhaps this is why things are so desperate in contemporary times. Maybe the Starborn from 666 years ago died young / went evil / etc, making him/her unable to take on the Gatekeeper/Fulcrum role, leaving Rashid to juggle things for much longer than he really could or should.

What are your thoughts? Any other clues here? What does this really mean in the bigger story?

With the grief that Harry has on Christmas Eve and him being in a happy relationship right now, it feels inevitable that Murphy's going to die. Which makes me booth dreading and looking forward to Battle Ground.

As for Peace Talks, I loved every scene, but in the end I felt really unsatisfied. There were so many Chekhov guns introduced that never went off/resolved (and will probably be coming into play in BG): Sanya's in town (flight delayed 7 times, LOL), lightsaber not cutting people, the Titan, Harry being kicked off the White Council, Ebeneezer's warning that Harry will be betrayed (he did kind of do that himself but I don't think that counts), why Thomas went assasin, Harry's conjuritis (which was almost done but I feel like isn't over yet), Rudolph's plot (does it even matter who he's working for at this point? The other fish to fry are so much bigger that it feels kind of irrelevant).

Some other ones, like the hint that Little Chicago survived, were subtle enough that it didn't bother me. I'd be fine with that not getting resolved for a few books; it's a nice teaser. Ditto with the favor that Harry owes Molly. That can be a long game.

I'm glad that Harry still has the Munstermobile. He's gotta have a silly car, and you just know that his flamed hearse is going to get comments at the peace talks.

On protection against Lara, it's hard to tell. Do you have to go all the way in sex in order to have True Love protection? The protection comes from intertwining your soul with the soul of your true love, right? And that definitely happens in sex, but also can happen to a lesser degree.
Back in another book (small favor?), Bob mentions that a hug between Harry and Murphy caused a little bit of soul swapping. Maybe protection against the whampires isn't an on/off thing, but a spectrum of how much soul swapping happened and how recently.