After starting the server, the first user joining the game generates this warning:

[07.03 16:42:36] [Server] [INFO] UUID of player Hap is 85f0078c-fc47-4c25-8c88-2d547a3c64b6
[07.03 16:42:36] [Server] [INFO] Hap joined the game
[07.03 16:42:36] [Server] WARNINGAn illegal reflective access operation has occurred
[07.03 16:42:36] [Server] WARNINGIllegal reflective access by com.comphenix.protocol.reflect.compiler.StructureCompiler (file:/plugins/ProtocolLib%204.6.1%20%23498.jar) to method java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int)
[07.03 16:42:36] [Server] WARNINGPlease consider reporting this to the maintainers of com.comphenix.protocol.reflect.compiler.StructureCompiler
[07.03 16:42:36] [Server] WARNINGUse --illegal-access=warn to enable warnings of further illegal reflective access operations
[07.03 16:42:36] [Server] WARNINGAll illegal access operations will be denied in a future release

Each login after that does not produce the warning.

Protocol dump: https://pastebin.com/qZ1Ze8WL