Healing Salve is considered the weakest of the five boons in Alpha. This was an ill omen for white, as many of its devotees feel that it has consistently been given short shrift of all the colors. Unexpectedly it also happens to be the most complex of the five boons, being both a modal spell and referring to damage prevention, a slightly more complex rules package than the other boons' abilities.
It is very weak, for example, compared to Lightning Bolt. Seven Lightning Bolts will kill your opponent, as opposed to seven Healing Salves which will only minorly frustrate your opponent. Lightning Bolt is also able to interact with creatures, whereas Healing Salve can at most transfer a trade from the creature to the Salve.
Healing Salve appeared in all core sets until Eighth Edition, but has only been reprinted once (in Duel Decks: Divine vs. Demonic) since then. For Dominaria, it was finally replaced by the strictly better Healing Grace. As a whole, partial damage prevention has been phased out as being mechanically unintuitive - Dominaria was in fact the last time partial prevention was printed.
Planar Chaos
Healing Salve was revisited in Planar Chaos as the green spell Healing Leaves, which is functionally identical to the original in all ways but the casting cost: 
instead of 
. Since life gain/damage prevention is less common in green than in white, Healing Leaves is not as grossly underpowered as Healing Salve, relatively speaking.
