RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Poisoned Steel Throwing Dart
The Old School RuneScape Wiki also has an article on Poisoned Steel Throwing Dart.

A Poisoned Steel Throwing Dart is a poisonous throwing dart. It is the third-weakest poisonous throwing dart in RuneScape Classic, and is made by adding weapon poison to a steel throwing dart. It has no requirements to equip, which may be a glitch, as the non-poisoned version requires level 5 Ranged to equip.

Like all throwing darts, they provide no WeaponAim or WeaponPower, and when they are equipped, they are invisible to other players.