Some what for Alliance too. So I'll start with them.
Very early on in Legion, Tirion, a famous Alliance Hero, was killed in such a way that it was like throwing the character to death.
The former king of alliance the same, though I can see how it could have been story development
Then there is Horde
Thrall pretty much out of the picture
Famous troll died in a stupidly done and lazy way with no creativity in thought...like it was rushed (which because his death was so rushed, helps lead me to conclussion)
Garrosh died (though he wasn't really a part of WoW until cataclysm)
That tauren leader died pretty early on
Sylvanas is a re-hash of Garrosh and like the troll guy (Vol'jin I think?), is going to be done in a VERY lazy and uncreative way. A copy/paste of Garrosh
Now one may think there is a vendetta against horde in blizzard. But alliance too have lost some, but not all. But it actually makes sense they are rushing these characters as fast as possible to death with no creativity in thought (the former king was the only creative one that required work and thought put into it with its own cinematic)
Sylvanas voice actor, a beautiful lovely happy women, is getting up there in age. Her contract is very likely to be running out, and she likely wants to retire. That or move onto other things
See where I'm going?
The troll voice actor for Vol'jin contract was also likely to expire, and either there was a money/budget issue with him, or/and they wanted cheaper voice actors to replace vol'jin or the actor just didn't want to voice him anymore or had/has other gigs going on
Thrall didn't even get a real exit storyline, he isn't even part of world of warcraft anymore. I'm guessing something went on behind the scenes, and they are trying to figure out how to make him stay on board...that or trying to find a good way to end the character so they don't have to pay the voice actor behind thrall, or any number of reasons involving budget/contract/money etc.
Tauren leader same, likely a contract issue of some kind and either blizzard didn't want to renew (for whatever reason) or the voice actor wanted to move onto other things
So it happens most of the horde voice actors are either contracts ending for whatever reason, want to retire or just have other things going on in their careers.
And it just so happens most of it coincides with horde characters. And alliance voice actors are (so far) more stable in their contracts, minus a couple famous heroes (like again, Tirion). But once their contracts become an issue, you too can expect alliance to lose most of their famous heroes as well.
It makes most sense why so many of these deaths (especially horde side) are done in a fast, uncreative, VERY rushed manner. Also why they are copy/pasting garrosh with sylvanas, because it was the fastest/cheapest thing to do before sylvanas's voice actors contract ends.
Also to add. You don't just kill someones character if they have a long-term contract with voicing that character. That also leads to that likely the contracts were expiring, so they had to kill the characters off.
Honestly, I hope this is the case. Cause that make it a tiny bit better about the rushed lackluster uncreative deaths most of these characters had, besides the former human king guy...varian or something. So he was most likely actually part of the story for his death...the others...not so much