On the Cedar stack, the slug is still read-only, but the ephemeral filesystem is writable. The slug is what gets deployed on each new dyno spawned. The ephemera

On the Cedar stack, the slug is still read-only, but the ephemeral filesystem is writable. The slug is what gets deployed on each new dyno spawned. The ephemeral filesystem is the individual file system on each dyno. So a plugin like WP Super Cache would be able to write to the file system, but that cache would only exist for the individual dyno that wrote it.