As announced here: (Staging Hierarchy Changes)
the staging root was updated yesterday to new roots. We use the staging roots for testing in our dev environments as described on the staging environment page, putting those roots in our trust store.
It's the same issue as New issuer for letsencrypt staging, something must have gone wrong during the maintenance. Our testing environments are busted as well.
This documentation page was an oversight during the improvements and changes to the Staging hierarchy. We will be working to get it fully updated today.
In the meantime, the Staging roots are available in similar naming scheme to the production Issuer URI.
We are working to update the chains served by the staging front ends and update the website to have all the certs so you can manually configure any chain you’d like. Thanks for your patience while we make these changes.
X2 certs (i.e. certs issued from our ECDSA intermediate E1) are not yet available, and the changes to staging which will make them available there have not yet happened either. That'll be a separate change.