The concept that eventaully became the Historic jewels was originally concepted as a way of corrupting the passive tree - Something a bit like Glorious Vanity, but with no way to know what effects it would have. It being non-removable once socketed in the tree was discussed.
It wasn't used at the time, but Hrishi and I liked the core idea enough that we kept discussing and refining it, and eventually Hrishi found it a home in Legion, and we expanded the concept so each civilisation could change the tree in different, usually more predictable, ways. I think this has ended up better than the original concept, personally.