Reliability: It’s not great

Would the simple solve be that Fly.io just mark any new service of theirs “beta” for x-months post launch?

None of the services that they have had issues with in the recent past are new, they have been running for at least a couple of years. They would need to put a "beta" sticker on the whole platform for that suggestion to work.

But the post makes it clear that the issue isn't that they had problems with new services. It was rapid customer growth before they had time to scale up the infrastructure as they had planned to do.