Why is there no classic mode?
They removed it, duh.
By the way, breaking news 
: Naxx has been released.
With that saidā¦
Itās the only good mode?
Well, it was one of, yes.
Well, there have been plenty of topics and discussions, for example: Introducing Hearthstoneās New Mode: Twist! - #35 by SparkyElf-2852.
Nope. Thatās what some forum geniuses, who were thinking that they were thinking (surely they were capable of it), were thinking 
, but yes:
As for the actual reason for the Classic mode, sadly, being discontinued, Iām gonna leave a link to this intevriew with the developers:
https://gamerant.com/hearthstone-twist-history-creation-how-why/
In particular:
Sadly, Hearthstone has decided to get rid of Classic and replace it with Twist.
Hearthstone is doing this for two major reasons. First, like the Mercenaries mode Hearthstone introduced in the same patch, Classic simply isnāt popular anymore. āUltimately, Classic is a static format. No new cards are added to it, the meta doesnāt change, and there are no new balance adjustments,ā London said, ābecause of the static nature to it, engagement waned over time.ā Though it had an initial spike in popularity, interest dropped quickly.
The second reason is that, with many cards being updated for modern Hearthstone, some cards act differently in Classic. London said, "One of our commitments for Twist is that weāre hoping there will only be one version of each card across all the formats.ā Now that Classic is gone, players wonāt have to memorize two copies of certain cards.
That bit about āthe same patchā (as Mercs) by the author of the article is a bit weird, but otherwise, thatās apparently it.