No built-in remote state backend storage (s3, gcs, etc) locking except their proprietary hosting. Obviously they need to make money, but this makes driving migration from terraform to pulumi a lot more difficult.
Alternately I was considering just implementing the server API to do state storage & locking, but we have implemented some workarounds that are good enough for now.
I think it's really unfortunate (whatever the reason) that the team was this slow to provide meaningful feedback on the PR.
Concurrent state locking to prevent corrupting your infrastructure state in a team environment