According to the release notes for 24w03a, server resource packs are supposed to be maintained across transfers. This is true if transferring while in the play phase, but not while in the configuration phase.

Steps to Reproduce:
Using a custom server:
1. Get access to a player in the configuration phase. This happens automatically during login, or can be triggered after login by sending the player a ClientboundStartConfigurationPacket.
2. Send the configuring player a ClientboundResourcePackPushPacket.
3. Await a ServerboundResourcePackPacket containing a terminal response from the configuring player.
4. Transfer the configuring player to another server by sending them a ClientboundTransferPacket.

Observed Results:
When the vanilla client receives the ClientboundResourcePackPushPacket, it downloads and applies the server resource pack as expected. However, when the vanilla client receives the ClientboundTransferPacket, it removes the server resource pack during the transfer.

Expected Results:
The vanilla client should maintain the server resource pack across the transfer.

Cause:
ClientConfigurationPacketListenerImpl overrides onDisconnect(), and calls Minecraft::clearDownloadedResourcePacks() regardless of whether the client is transferring.