SSLO Security policies; do we still need the Pinners category?
Certificate pinning was never intended for browser traffic.
In the simplest sense, modern browsers contain TWO CA trust stores - a system-level and separate user-level store, and a policy that says, basically, that a pinned certificate violation shall be ignored if the issuer is trusted via the user-level trust store. So in an SSL forward proxy, when you import the CA certificate to the clients, you're placing that CA in the user-level trust store, thus negating the effects of certificate pinning.
What is not covered, however, are non-browser agents that do certificate pinning. These are typically your antivirus and OS/software update agents. These non-browser agents have a single CA trust store and thus must honor all certification pinning validations. Without the pinners category in SSLO, these agents would break.