 The role of decoherence in QBism is rather downplayed. E.g. Fuchs and Schack (2012, Section 7) see it in light of the reflection principle (concerning an agent’s beliefs about their future beliefs). Specifically, in the context of a von Neumann measurement chain, an agent can use the state of the system as decohered by some later elements of the chain as an expression of their beliefs about what their beliefs will be after the previous elements of the measurement chain have been completed. (And of course, the results of decoherence can be taken into account if an agent is considering making measurements on a system that is in interaction with some environment.)