They have migrated to PostgreSQL due to performance issues.
> Unfortunately RethinkDB was a bit slow. It was fast enough for our needs at the time but as we grew we gave up waiting for its performance to be improved. We sponsored one improvement in an effort to help it scale with us but it wasn’t enough. We ended up abandoning it in favor of PostgreSQL.
It would be awesome to know if the other issues hey had have since been improved. RethinkDB has some great ideas and great ergonomics and I'd love to use it again.
> Unfortunately RethinkDB was a bit slow. It was fast enough for our needs at the time but as we grew we gave up waiting for its performance to be improved. We sponsored one improvement in an effort to help it scale with us but it wasn’t enough. We ended up abandoning it in favor of PostgreSQL.
https://www.zerotier.com/on-the-gpl-to-bsl-transition/