I went to try to get an assist from paper since there was so much connection nonsense in front of it I couldn't find a plugin to look at initially, but they found it pretty quickly... snippet of that discussion:
zAikarToday at 11:36 AM
@mr_coffee1026 remove Sound muffler plugin, its cause
mr_coffee1026Today at 11:36 AM
orly
zAikarToday at 11:36 AM
i tried to reduce risk of it last night but caused other bugs, ill try more later
that plugin is accessing world state asynchronously
not safe, and is what caused this
Things will run fine for a time then the server freezes up, even logs don't happen for a few moments before the thread dump begins. Since removing SoundMuffler, the server has run overnight now without doing this: https://pastebin.com/TtuHFXvP
I've been getting these "This is not a bug or a crash do not report to paper" insanity spam since just after Paper build 230. So up to and including build 230... SoundMuffler's approach was not an issue. I only saw anything about packet nonsense in build 244 (which I never ran), and which was reverted in 245 (which is where the above thread dump came from).