Author here. Crux [1] (MIT license) is currently still alpha but the supported beta release is arriving in September. We also know of two companies deploying Crux into production in the next week or so. We will be making further announcements during our talk at Strange Loop [2].
[1] https://github.com/juxt/crux
[2] https://www.thestrangeloop.com/2019/temporal-databases-for-s...