Hardly a new one, h5n1 first appeared in humans almost 30 years ago.
h5n1 with a couple of gene tweaks would be the near perfect means of culling half to 80% of the population. It already has that high of a fatality rate in humans no need for any edit there. It also can infect, birds, pigs, cows, and marine mammals so the food supply vector is covered. All that’s needed is the protein coat to make it survivable in aerosol form for a few hours time so it can go truly airborne not droplets but like measles or smallpox carried on submicron particles that every mammal breathes out just living. Given that is exactly what was done to covid and covid then had the highest R0 ever recorded it’s well within human technical means to add that to H5N1 wouldn’t be that hard a college level lab with the protein sequences could do it. You couldn’t come up with a more effective way to halve the population than that.