The 420 code was in a draft version of the WebDAV standards. Some programming languages have this code registered as "Method Failure". As such, it is unsuitable for use for a different purpose.
Instead, a version of this will be proposed as a RFC 9457 registered problem type. The problem type will provide information on the policy violated, the URL and any bearer token required to assert remediation. Resources
Test server that returns a 420 error code: https://httpstat.us/420 ITEF Draft tools
Author tool: https://author-tools.ietf.org
Internet draft submission tool: https://authors.ietf.org/submitting-your-internet-draft#:~:text=Internet%2DDraft%20submissions%20are%20made,will%20simplify%20the%20submission%20process