Prosperity - "Swiftmend now has 2 charges, and its cooldown is reduced by 3 sec."
Soul of the Forest - "When you cast Swiftmend, you gain Soul of the Forest, increasing the healing of your next Regrowth or Rejuvenation by 200%, or increasing the healing of your next Wild Growth by 75%."
There is obviously a lot of synergy between these two talents. Soul of the Forest seems pointless unless you also take Prosperity. Without prosperity, you are able to cast Swiftmend less than half as often, and only once every 25 seconds. Prosperity brought that cooldown down to 22 seconds, and since it gave you two charges, you could cast it twice within the span of a few seconds if you wanted. That made it far less punishing when you have to decide to use Swiftmend for the Soul of the Forest buff, or save it as an emergency heal. With Prosperity, you could use it for the Soul of the Forest buff and still have one charge left to use as an emergency heal.
In Shadowlands, Prosperity has been removed. Replaced by Nourish. I don't mind seeing Nourish come back, but what is the point of Soul of the Forest without Prosperity? Why would they remove Prosperity but leave Soul of the Forest? I know the Prosperity + Soul of the Forest build was not popular with everyone, but I made it work pretty well for myself.