Hi. When I created a world in pre-release 3 of Minecraft 1.16 for seeing the new Nether Fortress spawn rate, I went to some of them and all of them were infested of wither skelletons. There were always a minimum of 3 wither skelletons atacking me, and when I killed one another appeared. They spawn with a lot of frequency.

How to replicate:

• Create a new world
• Go to the nether
• Locate a Nether Fortress
• Fly around on creative or explore it in survival, you'll see a lot of our tall black-boned friend.