ReducedDebugBackport

A small Spigot/Paper 1.16 plugin that hides invisible players from the
visualize_entity_supporting_blocks debug renderer that newer Minecraft clients
(1.21.x) have, when they connect to a legacy server through ViaVersion.

⚠️ Important: reducedDebugInfo is all-or-nothing. For every affected client it
also hides F3 coordinates, hitboxes (F3 + B) and chunk borders (F3 + G) —
not just the entity-supporting-blocks renderer. There is no way to disable only that
one renderer. Limit who is affected with min-protocol-version and the
reduceddebugbackport.bypass permission.

The problem

Since 1.21.x the vanilla client exposes its internal debug renderers in a UI
(F3 + F6 → Debug Options → Debug Renderers). One of them,
visualize_entity_supporting_blocks, can be set to Always and draws a marker
on the block under every entity — including players hidden by invisibility.
It renders from data the client already has, so the server can't simply "turn the
button off".

The vanilla lever against it is the reducedDebugInfo flag
(gamerule "Limits contents of debug screen"). When active, the client disables
these debug renderers. The flag is delivered to the client in the login packet and
toggled at runtime via the Entity Status / Entity Event packet (status 22 =
enable, 23 = disable) targeting the player's own entity.

Compatibility

Works on Spigot/Paper servers 1.16 and newer.

The reducedDebugInfo game rule has existed since 1.8, so on any server this
plugin supports you can instead simply run:

/gamerule reducedDebugInfo true

for the same effect server-wide. Use this plugin only when you want
per-player control — e.g. hide F3 only for the newer ViaVersion clients that
actually have the visualize_entity_supporting_blocks renderer, while leaving
native players' F3 (coordinates, hitboxes) untouched. The
reduceddebugbackport.bypass permission gives you that granularity; the plain
game rule does not.

Requirements

• A 1.16 Spigot/Paper server.
• ViaVersion (used to detect the real client protocol version).
• PacketEvents or ProtocolLib (for sending the packet).

Installation

1. Drop ReducedDebugBackport-1.0.jar into plugins/ next to ViaVersion and
   PacketEvents (or ProtocolLib).
2. Start the server once to generate plugins/ReducedDebugBackport/config.yml.
3. Adjust min-protocol-version if needed and restart.

Configuration

config.yml:

# Client protocol version FROM which to start sending reducedDebugInfo.
# Clients BELOW this value are left untouched (they keep F3 coordinates/hitboxes).
min-protocol-version: 774   # 774 = 1.21.11

# Log every flag send to the console.
log-actions: false

# Verbose log: for EACH player prints the detected protocol version and the
# decision (SKIP / SENT / BYPASS).
debug: false

Common protocol numbers:

Protocol	Version	Protocol	Version
754	1.16.5	767	1.21 / 1.21.1
763	1.20 / 1.20.1	768	1.21.2 / 1.21.3
764	1.20.2	769	1.21.4
765	1.20.3 / 1.20.4	770	1.21.5
766	1.20.5 / 1.20.6	772	1.21.7 / 1.21.8
773	1.21.9	774	1.21.11

Command

/rdb [off] — operators only. Diagnostic helper:

• Prints the detected protocol version, the threshold, the bypass state and the
  "normal decision" (SEND / SKIP).
• Force-sends the packet ignoring the threshold: /rdb sends status 22
  (enable), /rdb off sends status 23 (disable). Useful to confirm the packet
  itself works regardless of version detection.

Permission

Permission	Default	Effect
reduceddebugbackport.bypass	false	Player is ignored — reducedDebugInfo is not sent, full F3 is kept.

The bypass is checked at apply time (join / respawn / world change), so changing it
for an online player takes effect after their next respawn or relog.