If you google "oversteeping tea benefits", there are claims that the health benefits increase when more antioxidants and caffeine are released from longer steeping. Livestrong has this article: https://www.livestrong.com/article/2851 ... eping-tea/
It may get more bitter from additional caffeine and tannins, but those things are not necessarily bad for you. I almost always oversteep, and I like the stronger taste. I guess it is individual choice.
Made from leaves that have not been oxidized.