I feel like there was a batch of big software from the 90s and early 2000s that took a really long time to die, and they're not necessarily dead yet: Windows XP. Java. Python 2. IPv4. X11. Free/Open/NetBSD (1). Hell, I was using (adding new data to) a MUMPS-based system on OpenVMS until 2018. But I feel like we are seeing fewer new widely-publicized death marches now, despite many more projects starting. What changed…
Usenet activity is a very odd measure of deadness to use today.
Although maybe that is exactly what you mean: that the BSDs were already dying in the Usenet era and are still not dead? I think even if they were dying back then, over the course of the past 20 years they may have found their niches to heal and survive in.