Hello!
There is a guaranteed legendary card from every first 10 packs.
Does it apply to class packs?
I think so. I got a legendary card both from mage and hunter packs.
https://hearthstone.gamepedia.com/Card_pack_statistics#Pity_Timer
says it does.
ecore100 (twitch and youtube streamer) recently started a new f2p, climb-to-legend account, and got an Edwin VanCleef very early (might have been his first pack).
Edit: Oops. I read “Classic packs”, not “class packs.”
If I’m not mistaken developers said “no”.