So wondering why everything died, I contacted server admin.

By everything I mean every tamed horse, item frame (with item inside, and without) fish, golem, villager, minecart (with and without hoppers) AND PLAYERS, etc. In all active chunks (both nether and overworld)

Apparently the admin used a command "kill @e[type=!minecraft:zombie_pigman]"

and instead of the desired effect: killing zombie pigmen to check an area, it literally killed ... everything.

Now I don't use commands so I have no idea but a small search shows "kill @e[type=zombie_pigman]" as the correct method to do the desired effect...

I list this as a bug because the first method (!minecraft:zombie_pigmen) should throw an error or invalid type or something, this was quite a disaster and blunder on the admin's part, and if not in a snapshot/test world, could have caused a lot more damage. I mean it's a pretty honest mistake as the game lists things like that but to simply target literally all entities in active chunks is a bit of an oversight. (you'd think there'd be a command like type=all)