Addon: AFK Chair
Author: koidev_

AFK Chair makes inactive players immediately recognizable. After a configurable period without movement or actions, a chair appears automatically, seats the player and displays an AFK timer with sleeping particles.
World owners can adjust how long the addon waits before activating and can enable optional protection for an inactive player. The settings are available through a simple in-game menu.

Returning is just as direct: jumping or crouching removes the chair and restores normal control. The system is useful on multiplayer worlds where other players need to understand that someone has temporarily stepped away.
The configurable delay and protection setting make the mechanic suitable both for private worlds and for active multiplayer servers.
How to install the add-on
Download the .mcaddon or .mcpack file, open it with Minecraft and wait for the import to finish. Then enable the resource and behavior packs in the world settings. If the add-on requires experiments, turn them on before launching the world.