I learned recently about RFC 7710 which specifies a DHCP (v4/v6) and RA option for "You're on a captive portal, here's the website you should visit before you get access": https://tools.ietf.org/html/rfc7710

Do any of the major implementations of captive portals support it?

RFC 7710 is only a proposed standard.

It has been implemented on a few captive portal solutions[1] but as far as I know it is not understood by any client software.

[1] https://github.com/coova/coova-chilli/pull/274

RFCs are never anything other than proposed standards, or informational documents. There is no point at which an RFC becomes a “recommended standard” or some such thing.

All internet standards, from IP to HTTP, are proposed standards, it doesn’t actually mean anything about whether they’re generally implemented or not.

Actually, they do become standards:

https://www.rfc-editor.org/standards#IS

OH yeah, that started happening recently, I forgot about that.

Not that I know. My pfSense firewall doesn't have it (IIRC), so my guess would be that poorly maintained router boxes in a hotel basement definitely don't have it.

I'm not sure if the various DHCP clients communicate this properly to the OS or browser even (I wouldn't know how to query for it on Linux)

If you're using NetworkManager you can get DHCP options by being mildly angry at the D-Bus API:

    $ python3
    >>> import dbus
    >>> bus = dbus.SystemBus()
    >>> nm = bus.get_object("org.freedesktop.NetworkManager", "/org/freedesktop/NetworkManager")
    >>> conn = bus.get_object("org.freedesktop.NetworkManager", nm.Get("org.freedesktop.NetworkManager", "PrimaryConnection", dbus_interface="org.freedesktop.DBus.Properties"))
    >>> dhcp = bus.get_object("org.freedesktop.NetworkManager", conn.Get("org.freedesktop.NetworkManager.Connection.Active", "Dhcp4Config", dbus_interface="org.freedesktop.DBus.Properties"))
    >>> options = dhcp.Get("org.freedesktop.NetworkManager.DHCP4Config", "Options", dbus_interface="org.freedesktop.DBus.Properties")
    >>> str(options["subnet_mask"])
    '255.255.255.240'

I guess you can parse /var/lib/dhcp/dhclient.*.leases otherwise?