Of note, just today Jagex released a plugin interface for Runescape 3 which was developed together with developers from the Runelite team and the existing Runescape 3 third-party plugin tool (Alt-1). The relationship seems pretty friendly right now.

Given that Jagex (and whoever owns them now) can pull the rug and kill Runelite at any time, it's remarkable there's been some sense of harmony at all. Concessions have been made both ways.

The relationship isn't perfect. But the fact that there's a relationship and a two way dialogue at all? Unheard of.