I used trace to figure out what packages were modifying serverCmdMessageSent, and deactivated all of them, and chat still doesn't work
EDIT: Chat works for non-admins only.
The message is being echoed to the server console, regardless of admin status, but it's not being sent into chat if client.isAdmin or client.isSuperAdmin are true
EDIT2: Spam filter is also broken, nonadmins can spam as much as they want
It seems an add-on did break it, though deactivating the package didn't fix it. I had to disable it and restart the server.