  Decaying Cold Dark Matter (DCDM) is a model that is currently under
investigation regarding primarily the $S_8$ tension between cosmic microwave
background (CMB) and certain large-scale structure measurements. The decay into
one massive and one (or more) massless daughter particle(s) leads to a
suppression of the power spectrum in the late universe that depends on the
relative mass splitting $\epsilon=(1-m^2/M^2)/2$ between the mother and massive
daughter particle as well as the lifetime $\tau$. In this work we investigate
the impact of the BOSS DR14 one-dimensional Lyman-$\alpha$ forest flux power
spectrum on the DCDM model using a conservative effective model approach to
account for astrophysical uncertainties. Since the suppression of the power
spectrum due to decay builds up at low redshift, we find that regions in
parameter space that address the $S_8$ tension can be well compatible with the
Lyman-$\alpha$ forest. Nevertheless, for values of the degeneracy parameter
$\epsilon\sim 0.1-0.5\%$, for which the power suppression occurs within the
scales probed by BOSS Lyman-$\alpha$ data, we find improved constraints
compared to previous CMB and galaxy clustering analyses, obtaining $\tau\gtrsim
18$ Gyrs for small mass splitting. Furthermore, our analysis of the BOSS
Lyman-$\alpha$ flux power spectrum allows for values $\tau\sim 10^2$ Gyrs,
$\epsilon\sim 1\%$, that have been found to be preferred by a combination of
Planck and galaxy clustering data with a KiDS prior on $S_8$, and we even find
a marginal preference within this regime.
