With Multiconnect enabled, all other mods I have installed no longer receive custom payload packets.

Affected mod combinations in my setup:

Servux (server) + Minihud (client) - structure bounding boxes not showing
WorldNamePacket (server) + VoxelMap (client) - world name packet not received by VoxelMap

This is intentional. I made a whole API to get around this. Use it.

Not blocking custom payloads from other server versions is a recipe for disaster, and has already caused multiple issues back when multiconnect didn't block them. It's better for mods to explicitly acknowledge that their assumption that the server version is the same is broken, then they can still communicate to the server using the multiconnect API.

Thanks for the explanation, I didn't think about this scenario. I only now realized it works correctly when the version matches, so it's not as a big problem as I thought.