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Reliable processing of quantum information is a milestone to achieve for the deployment of quantum technologies. Uncontrolled, out-of-equilibrium sources of decoherence need to be characterized in detail for designing the control of quantum devices to mitigate the loss of quantum information. However, quantum sensing of such environments is still a challenge due to their non-stationary nature that in general can generate complex high-order correlations. We here introduce a path integral framework to characterize non-stationary environmental fluctuations by a quantum probe. We found the solution for the decoherence decay of non-stationary, generalized Gaussian processes that induce pure dephasing. This dephasing when expressed in a suitable basis, based on the non-stationary noise eigenmodes, is defined by the overlap of a generalized noise spectral density and a filter function that depends on the control fields. This result thus extends the validity to out-of-equilibrium environments, of the similar general expression for the dephasing of open quantum systems coupled to stationary noises. We show physical insights for a broad subclass of non-stationary noises that are local-in-time, in the sense that the noise correlation functions contain memory based on constraints of the derivatives of the fluctuating noise paths. Spectral and non-Markovian properties are discussed together with implementations of the framework to treat paradigmatic environments that are out-of-equilibrium, e.g. due to a quench and a pulsed noise. We show that our results provide tools for probing the spectral and time-correlation properties, and for mitigating decoherence effects of out-of-equilibrium -- non-stationary -- environments.