Part yes, and part no. Of the characters you mentioned…
Saurfang’s character “development” was obviously very controversial. It’s also however a moot point… because he’s dead.
Baine is almost universally disliked, and he didn’t even really develop past “Jaina’s Best Friend” which he really already was. In less memey terms, we already knew that Baine was an Alliance-leaning, peace-first leader. He was consistent to form - a form which not many players like.
Zek’han wasn’t really developed at all. He shows up twice with minimal dialogue. He will GET development in the upcoming book, but it takes more than that to endear a character to players. Like Sarm said, it feels like they latched onto the popularity of a minor figure and are just trying to cram him in the players’ faces, rather than let him show up and do some stuff and let that relationship grow a little bit more organically.
Voss is probably the best-developed and best-received of the characters you’ve listed. That’s being generous and ignoring the fact that her character did a hard 180 from previous characterization, but the end result seems to have worked out, so I’m not going to complain.
Rastakhan got great development and was liked by a lot of Horde players and… oh yeah, he’s dead.
Talanji got lots of screen time, but didn’t change essentially a single iota from the Precocious Princess she was presented as from the start.
Vol’jin… I don’t really feel like he was developed as much as he was set up for future development, which he seems set to receive.
There’s one thing I will say, however. “Development” is often a coded way of referring to a character being out there on the map, doing stuff in the story. Umbric is getting accolades in this thread because he was present in the Alliance war campaign fairly often, was doing stuff for the Alliance fairly often. He was a visible presence - you can even see him doing his dinosaur thing when playing Horde during invasions. (He’s right next to one of the neutral flight points).
The Horde war campaign features lots of Nathanos - oh yeah, he’s gone now too - and Baine, with a smattering of Rexxar and bit appearances from some other characters like Valtrois - who is, oh yeah, comparing the Horde to the Legion when you see her. So, part of it is that a lot of character development went into characters that didn’t see the end of the expansion, part of it is that character development went into characters who are by nature passive actors, and another part of it is that the very nature of the Horde story sucked a lot of the oxygen out of anything that wasn’t the main narrative.
We didn’t get any development for main characters like Tyrande or Jaina do, we didn’t get memorable fun new minor characters like Flynn Fairwind or Taelia, and the development we did get for characters like Thalyssra or Baine was entirely unflattering. (Thalyssra has been done super, duper dirty by the narrative, and she’s barely done anything wrong - they just make her look like a jobber any time she shows up.)
I think the kind of characters a lot of Horde players are looking for are something like Occuleth. (Bear with me here). He’s quirky, he doesn’t make grand moralizing statements and isn’t used as a stand-in for whatever message the writers want to convey, and he has something that he’s good at and is visibly doing for us often. In short, they aren’t trying too hard with him, and he’s a decent minor character who is fun when he shows up. He also isn’t dead, which is a plus. Chadwick Paxton’s Forsaken squadron is another example of this.