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Heavy sterile neutrinos can be produced in core-collapse supernovae (CCSNe), which are superb particle generators because of their high densities and temperatures. If the sterile neutrinos are long-lived, these may be produced inside the supernova core and escape the stellar envelope, later decaying into SM particles like photons and neutrinos. In this work, we first improve the original analytical calculation of the γ-ray fluxes from decays. We then revisit the bounds on the sterile neutrino parameter space from the non-observation of γ -rays from SN1987A by the Solar Maximum Mission (SMM) and constraints from the diffuse γ -ray background arising from sterile neutrino decays. We find that the constraints arising from both the SMM data and the diffuse γ -ray background are weaker than those that have previously appeared in the recent literature. Finally, we study the sensitivity of several present and near-future γ -ray telescopes such as e-ASTROGAM and Fermi-LAT, assuming a nearby future galactic CCSN. We show that future observations can probe mixing angles as low as |U_τ/μ4|²∼ 5 × 10^-17.