Servers don't really matter much no besides for playing with friends/guild. Go here and see which server you get the best ping on and matches your local time.
Blackrock used to be great, Kil'Jaeden is where my friends and I are playing right now, it's a pretty great server, I don't know what you mean by dead. Tichondrius is pretty toxic, always has been since the big surge in population 4 or so years ago due to popular YouTubers/Streamers playing on there. Plus everyone there thinks they are good when in reality just they are all "stuck" at 1900 rating because they "always have bad teammates".
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Because all the shitter players flocked to Alliance to min/max with their racials, while all the good players who didn't care about stuff like that stayed Horde.
Kil'Jaeden is pretty awful. I played there since vanilla and it's a pretty much a medium population horde pve server.
It is dead if you compare it to bigger servers.