Hey author here, thanks for reading the post. I haven't seen UUIDv5 used very often but I'll have to think about the idempotency angle more.

One thing that concerns me is that UUIDv5 is based on SHA1, which is an insecure hashing algorithm. I generated a pair of colliding UUIDv5s in an earlier post in this series (https://alexsci.com/blog/uuid-oops/#insecure-hash-functions). So I don't think you can use UUIDv5 for security critical functions.