Describe the bug

I get a "unknown packet serverbound/minecraft:chat_session_update" error from the client, when connecting to a velocity instance with the plugin, where the backend server does not have the plugin.

It would be useful to not require the plugin on the backend server for performance/simplicity reasons. additionally, it might be impossible to install the plugin on the backend server is not a flavor of bukkit/spigot/paper/etc. (say for example vanilla, fabric, or forge)

To Reproduce

Install the plugin on a velocity proxy instance, but not on the backend server.

Expected behavior

Custom packets to not be sent through the proxy to the client, causing a connection termination.

Operating System

Linux Mint

Server Software

Velocity 3.3.0-SNAPSHOT (git-00ed2284-b415), paper-1.21.1-33

Server Version

Not Listed

Proxy Type

Velocity

Plugin List

> velocity plugins
[12:05:45 INFO]: Plugins: antipopup, pistonqueue, skinsrestorer, viaversion, viabackwards

> plugins
[12:05:39 INFO]: Server Plugins (4):
[12:05:39 INFO]: Bukkit Plugins:
[12:05:39 INFO]:  - AntiPopup, PistonQueueBukkit, ViaBackwards, ViaVersion

> plugins
[12:06:15 INFO]: Server Plugins (4):
[12:06:15 INFO]: Bukkit Plugins:
[12:06:15 INFO]:  - AntiPopup, LuckPerms, ViaBackwards, ViaVersion

Server logs

https://gist.github.com/bradcarnage/9d94ac5899c3676efe43db0923b34e75

AntiPopup only blocks the popup on velocity. It needs to be installed on all backend servers as well.
You shouldn't worry about performance, packetevents handles packets asynchronously.

For modded servers use a mod to handle blocking chat reports.