Let me highlight arguably the most important line in the article:
Starting in Patch 26.6, coming later this month, Hearthstone will be replacing the Classic Mode with a new Mode called Twist!
Shouldn’t have bothered reading the rest. 
But anyway, back to the matter.
First, discontinuing support for Mercenaries (you could still do them if you haven’t, though), then this? Apart from some solo adventures, which have recently been made available for gold (beware, some of them have critical bugs: The Dalaran heist, Jepetto Joybuzz , also reportedly Adventure boss not ending his turn ), in my view, there’s not much left to play, then!
Well, I suppose it’s time to move on to something else, and this is also a relief of sorts: while Classic was fine, it has been somewhat less fun for me lately, bots and the like not helping, either.
Perhaps I could still do some Mercs tasks and stuff out of habit (or should I?), there are also some of the aforementioned adventures left to be done for me, but apart from that — I’m off, I suppose. As said, it’s a bit of a relief — time to close this chapter and move on to something else, at least thanks for sending a clear message that I should do it!
PS This post has turned out quite subjective, but I hope nobody minds that.