Resto Shamans have a love/hate relationship with mastery. It is great if you are working on progression while also underhealing a fight. But throw a couple disc priests or an extra healer into the mix, and mastery is pretty useless unless things go sideways and the raid is about to die from some mechanic a raid team member screwed up.
Shamans have been hoping for a change to mastery for ages, and it kind of looks as though Blizzard doesn’t have intentions of changing it at this time.
IMO, Deep Healing would be good as a situational/optional talent, but too situational for a mastery
Deep Healing is indeed situational. Strong for healing people that need healing. Weak for healing people that don’t. (Source)
Having Deep Healing as a talent would be great, so Shamans could switch it up when the raid is running down a healer or on progression. However, with stats being the way it is next expansion, it would literally require a second set of healing gear – one for mastery and one that will likely be haste and/or crit. However, if mastery continues to be the go-to stat for Elemental Shamans, it could get use as a second spec gear set for Deep Healing and/or Elemental Shaman.