It is a historical standard because it was implemented in the BSD inet_ntoa and copied into other implementation.

As was the long decimal format.

No RFC describes the long form IP address.

As none describes the dotted quad form IP address.

The RFCs I know that describe grammars for IPv4 addresses only support dotted quad form. This includes URLs.

They are all protocols. Protocols using IP don't define IP. Note by the way that the RFC for URLs (1630) defines host as digits . digits . digits . digits, thus allowing 999.123.0.12345. They require a quad dotted decimal address, but it doesn't say anywhere that that address is an IPv4 address. (Or perhaps I missed that specification)

most web browsers parse out the host portion of the http URL and pass it to inet_aton.

That is exactly what I suggest everyone should do. I've found it hard to find a tool on my Linux box that doesn't think 0x7F000001 is invalid. You talk about expectations. I think doing what other tools do lives up to people's expectations.

Including the long form IP addresses in a regular expression makes them much more complicated.

I'm suggesting that no regex be used.