  The stability of particles in the cosmic soup is an important property that
can affect the cosmic evolution. In this work, we update the constraints on the
decaying cold dark matter scenario, when the decay products are effectively
massless. We assume, as a base case, that all of dark matter is unstable and it
can decay on cosmological time scales. We then extend the analysis to include
the scenario where only a fraction of dark matter is unstable, while the
remaining part is composed of the standard, stable, dark matter. We consider
observations of cosmological probes at linear scales, i.e., Planck 2018 cosmic
microwave background temperature, polarization, and lensing measurements, along
with geometrical information from baryon acoustic oscillation (BAO)
measurements from SDSS DR7, BOSS DR12, eBOSS DR16 and 6dFGS, to derive
conservative constraints on the dark matter decay rate. We consider these
dataset separately, to asses the relative constraining power of each dataset,
as well as together to asses the joint constraints. We find the most stringent
upper limit on the decay rate of decaying cold dark matter particles to be
$\Gammadm < 0.129 \times 10^{-18}\,\mathrm{s}^{-1}$ (or, equivalently, the dark
matter lifetime $\taudm > 246$ Gyr) at 95\% C.L. for the combination of Planck
primary anisotropies, lensing and BAO. We further explore one-parameter
extensions of our baseline DCDM model. Namely, we vary the sum of neutrino
masses, the curvature density parameter, and the tensor-to-scalar ratio along
with the DCDM parameters. When varying the tensor-to-scalar ratio we also add
data from the BICEP/Keck experiment.
