That RFC is obsoleted by https://datatracker.ietf.org/doc/html/rfc9844 which removes all guidance around URIs:

> This document completely obsoletes [RFC6874], which implementors of web browsers have determined is impracticable to support [LINK-LOCAL-URI], and replaces it with a generic UI requirement. Note that obsoleting [RFC6874] reverts the change that it made to the URI syntax defined by [RFC3986], so [RFC3986] is no longer updated by [RFC6874]. As far as is known, this change will have no significant impact on non-browser deployments of URIs.

Fair enough, but that leaves us with no way to represent zone IDs in URLs at all. Neither http://[fe80::4%eth0]/ nor http://[fe80::4%25eth0]/ is valid under RFC 3986.

Given that net/url has supported RFC 6874 since before RFC 9844 came along, our choices are:

* Keep supporting the RFC 6874 syntax.

* Drop support for it, require strict RFC 3986, have no support for zone IDs in URLs at all. Breaks existing users, utterly infeasible.

* Stop supporting RFC 6875 and start supporting an unescaped % as the zone ID separator, which conforms to no standard I know of. Also breaks existing users, infeasible.

* Some sort of hybrid where we try to support both %25 and % as a separator? Ugh.

Of these, keeping the existing support as-is until or unless a new standard comes along seems like the best option.