Until recently, the staging environment used the same “happy hacker fake CA” issuer as the dev environment, whose private key was available in the Boulder repo. We recently generated a separate set of certificates and keys for staging. Now the staging intermediate is “Fake LE Intermediate X1”. Keeping the private key for staging secret allows us to submit to the “testtube” CT log from staging.
However, I think that might not be what you’re asking. It sounds like you’re asking: If you generate a certificate from the staging environment, is it a violation of the Subscriber agreement to reveal its private key. The answer is no - you don’t need to agree to the subscriber agreement to use staging (though I should check the config there), so it’s fine to reveal private keys for the untrusted certificates you get from staging.
However, keep in mind that anyone with access to the private key can revoke a certificate, both on prod and on staging.