It actually isn’t a war crime. Its legality is intentionally vague as it has numerous applications.

White phosphorus munitions are not banned under international law, but because of their incendiary effects, their use is supposed to be tightly regulated.[55] Because white phosphorus has legal uses, shells filled with it are not directly prohibited by international humanitarian law. Experts consider them not as incendiary, but as masking, since their main goal is to create a smoke screen.[56]

Source: https://en.wikipedia.org/wiki/White_phosphorus_munitions#International_law

Edit: some of yall need to take an English course or something. Me explaining how WP is intentionally kept in a legal grey area is not me actively supporting its use. Get a life dweebs.