NPCs
Retired addon — now a first-class construct
The npcs addon has been replaced by the built-in npc declaration: npc "name" { location, name, skin, look_at_players, on_click(player) { ... } } with set npc / remove npc statements — the spawn packets and the head-look trigonometry now live in a dedicated Java runtime, and click handlers are inline typed bodies. The old import "npcs" API and its spawn_npc / on_npc_click functions are gone.
Everything the addon did — skins, click handlers, nearest-player head tracking — is now one declaration the compiler checks. See the first-class NPC reference for the construct and its statements.
The retired addon was ~400 lines of SwoftLang over the raw packet surface: player-info + spawn packets for the entity, a polynomial atan2/sqrt for the head-look, and a closure-chain registry for state. Those techniques are still teachable on their own — the writing-an-addon tutorial and the raw packets reference cover the packet and registry patterns — but the NPC itself is no longer something you assemble by hand.