I'm self-hosting Pulumi with an S3 bucket as storage. The state of my current stack is only about 2.1MB, but the cumulative backups and history are growing to over 400MB. Is there any way via the pulumi CLI line to purge this history? Or will I have to write some scripts to purge this manually?

I do not think there is anything like that. Probably worth checking the public roadmap. There seem to be a track for improvements of other backends https://github.com/orgs/pulumi/projects/44/views/1

You could use S3 lifecycles to handle this. Got to be careful not to delete current-but-really-old files though...

Running into this myself, it's unfortunate there is no way to deal with them easily short of writing our own scripts