WotC hates Ajani because as a big cat he isn't relatable.
I mean, that might explain why he got demoted out of the "main white planeswalker" spot that Gideon took and went permanently GW, but it doesn't really explain the idea he's on the chopping block.
The PW that are likely to get killed are usually fairly obvious from how they're presented - Elspeth's death was set up a lot more given that she was a tragic hero in a block about Greek Mythology, but there's also the fact that Elspeth probably isn't dead-dead, just mostly-dead. Venser may have been a bit of a curveball, but someone like Xenagos or Domri or Vraska come off as complete throwaway characters that nobody would care if they got snuffed. Ajani doesn't really fit in that group - he has a lot of backstory, and they still create lots of cards around him, e.g. Jazal Goldmane.
Of the existing characters, I'd say the likelihood of them getting killed:
Ajani - Lots of backstory, unique race. Not gonna die.
Arlinn - Reasonably popular
Ashiok - Too interesting to kill off; works behind the scenes.
Bolas - Main Villain.
Chandra - Generic Red Walker/peppy girl with tsundere tendencies. Not getting killed.
Dack - Popular supplemental character
Daretti - Popular supplemental character
Domri - Total filler character, could get killed.
Dovin - Could get killed, he's an annoying miniboss for Tezzeret who doesn't have all that much space to do anything with, plus his card sucks.
Elspeth - She's dead, but coming back.
Freyalise - Permanently Dead.
Garruk - I think they will kill him off whenever they get ready to kill Liliana.
Gideon - I think they really like having Captain America on the squad.
Jace - Poster Boy for Magic.
Karn - He's gotten saved enough times I don't think he'll ever get killed.
Kaya - They're not going to kill Kaya anytime soon because they don't want to invoke the "token black character gets killed" trope.
Kiora - They only have one Merfolk Planeswalker.
Koth - He'll come back, but he strikes me as the kind of PW that's perfect to get killed off when they do bring him back.
Liliana - I think she's the Gatewatch member mostly likely to get offed if and when they get around to killing all of her literal demons.
Nahiri - Interesting villain, I don't think they take her out.
Narset - Barely did anything with her and they already killed her once.
Nissa - They went out of their way to build her up as perfect shy elven waifu. There's no way she ever gets killed.
Nixilis - Generic villain guy, they need a couple of these around, I think.
Ral - Killable, but they'd have to replace his niche as the Izzet spells guy.
Saheeli - Again, I don't think they'd kill Saheeli for PR reasons - introducing a specific minority character only to kill them comes off bad.
Sarkhan - I could see them killing Sarkhan off, actually.
Sorin - It depends how much they want to kill "Brooding Vampire Sephiroth who spends his off-time in a loud club blasting Rob Zombie music while he drinks blood out of a chalice." I don't think they do, even if I don't like Sorin.
Tamiyo - Not unless they *really* want to scour the multiverse of all evidence of Kamigawa's existence.
Teferi - Very killable. He's not even a PW at the moment, but he's basically Venser - a legacy character who no longer serves any purpose in the plot.
Tezzeret - Interesting minion character, not gonna kill him.
Tibalt - I think a lot of what they do with Tibalt will get reconned so they can have a Rakdos walker in Standard.
Ugin - I think he gets killed again, eventually. He's probably too powerful unless he turns out evil.
Venser - Already permanently dead.
Vraska - Very much a filler character that could get killed off.
Xenagos - Already permanently (?) dead.