Thanos is probably one of the other popular choices. It's being heavily used in production by a number of companies, but I don't think they've branded it at "Prod ready" in a 1.0 release though.
Thanos doesn't have production support for local storage either. The only stable storage providers for it are google, amazon, and azure's object stores.
Interestingly, it looks like Cortex's support for local storage and object stores comes from using Thanos's storage engine. So once it's production ready in Thanos it will probably be production-ready in Cortex shortly thereafter.
I may have misinterpreted what they meant by local storage! I was reading that as having a local copy of the TSDB available to Prometheus, (eg: how Thanos works) versus Cortex which doesn't store metrics locally (IIRC).
What you said is correct and makes sense. Though, I would suspect either choice works with any S3 compatible API that can run on local storage, but I know that isn't necessarily what's meant by "local storage".
https://thanos.io/storage.md/
Interestingly, it looks like Cortex's support for local storage and object stores comes from using Thanos's storage engine. So once it's production ready in Thanos it will probably be production-ready in Cortex shortly thereafter.
https://cortexmetrics.io/docs/operations/blocks-storage/
I think for Cortex your safest storage options now are Bigtable, DynamoDB, or Cassandra.