Here is the paper discussing J0931+0038, referenced in post #16.
In the conclusions, the authors state their might be a neutron star or black hole that becomes evident after more time passes.
"Though we focused here on the nucleosynthetic implications, we speculate that J0931+0038’s unique composition implies that the rare supernova events that could explain this signature should also be found in upcoming large transient surveys, such as Rubin/LSST (LSST Science Collaboration et al., 2009). If the chemical signature is due to an unusual PISN, the heavy Fe peak and neutron-capture elements point to the presence of a neutron star or black hole remnant involved in the explosion, which may result in unusual observational features of slowly evolving superluminous supernovae (Gal-Yam, 2019; Nicholl, 2021)."