Can star craft 2 heart of the swarm still playable?
Yes. At least for me it is.
If you mean the campain, you need to buy it.
If it is for multiplayer, well… the official public matchmaking is unfortunately in LotV crappy mode. But if you create custom game lobbies in melee multiplayer, they can be created also in WoL or HotS mode and the multiplayer will proceed accordingly too.