Minecraft wiki entry for Development_versions, associated text: Readded, previously a part of game known as "snooper" until 18w20c.
Only implemented in world load event now.
Sent data when loading singleplayer world or connecting to multiplayer server.
Contains following information:
Launcher identifier
User identitifer (XUID)
Client session ID (changes on restart)
World session ID (changes per world load, to be reused for later events)
Game version
Operating system name and version
Java runtime version
If client or server is modded (same information as on crash logs)
Server type (single player, Realms or other)
Game mode
Cannot be disabled. [23]