Yeah, just installed D3, got a little bored with D4 Season 1, and can confirm D3 is still online only and always game.
It's just that Blizzard "borrowed" a term from Path of Exile and in this last season you can create Solo Self-Found hero, meaning you can't play with others, join groups, trade, etc., just like in PoE!
But, it's still online and after season ends sometime in the future, that character will become "normal" again, just like in PoE, so cheating is still not possible and even if you try, ban is 100% certain!
😉