Does the Pharrah - Mercy combo is still usable ?

Well, it all depends on which ELO range you're playing; It's still very powerful across the board, but if you're at a Masters/Grandmasters level of play, their hitscan is going to take it down, but keep in mind; The Pharmercy duo will also have good gamesense/mechanics. It all scales, I suppose.

Generally; It's a very powerful duo to run and it's 100% still viable.