The claim is that a supernova about 150 light years from earth triggered a mass extinction about 2.6 million years ago which, among other things, killed off the Megalodon.
But then where is the supernova remnant? It could only have moved at most couple of thousand light years in that time (and really it should be orbiting the center of the galaxy roughly along with the sun anyway) so where is it? https://en.wikipedia.org/wiki/List_of_supernova_remnants
The technical paper suggests in its abstract that this supernova was one of a series which formed the Local Bubble of low-density interstellar medium. To a distant observer, the density change and star-forming regions at the edge of the Local Bubble might very well look like the large supernova-remnant nebulae we see from Earth. That is, we might be living inside the supernova remnant.
Whether there is a compact remnant as well, like a neutron star, is harder to say. Not every supernova is expected to leave a compact remnant. A black hole without a partner star is quite challenging to observe. And while we can frequently see neutron stars which are rotating and have bright spots on their surfaces, so that they "pulse," a neutron star whose beam didn't happen to point in our direction might likewise be nearly invisible.
