It does exist, it has been approved by the FAA, and you can go buy the STC right now if you want[0]. The incentive is that 100LL has been banned by at least one county[1] with states[2] trying to follow (although the AOPA successfully neutered that one[3]).

[0] https://www.g100ul.com/ [1] https://www.globalair.com/articles/california-county-that-ba... [2] https://www.flyingmag.com/washington-bill-seeks-statewide-ba... [3] https://www.aopa.org/news-and-media/all-news/2023/march/16/a...