305 Use Proxy - Redirection Response Status Code
Deprecation Notice
Part of a prior version, a 305 indicated that a requested response must be accessed by a proxy. This response was deprecated due to security concerns regarding in-band configuration of a proxy.
Official Definition
The 305 (Use Proxy) status code was defined in a previous version of this specification and is now deprecated (Appendix B).
Source: RFC 7231 Section 6.4.5