Hi Mohammed,
To my best knowledge, IRDP was never widely deployed. It was an add-on to the IPv4 stack, not a mandatory component, and so it required extra drivers on clients to be useful. Nobody seems to have bothered at that time to implement them.
You are correct that today's IPv4 clients can learn about their default gateway from DHCP. However, DHCP was first standardized in RFC 1531, 2 years after IRDP RFC 1256, so in times of IRDP, DHCP wasn't a mature technology yet.
If you look closely, IRDP principles are very similar to what we are using today in IPv6 - the Router Advertisement and Router Solicitation messages in IPv6 are very, very similar to the IRDP from IPv4. However, in IPv6, their support is mandatory, and as you surely know, hosts in IPv6 discover their gateway based only on Router Advertisements. DHCPv6 does not have an option for the default gateway address. Even if using DHCPv6, a host must learn about its default gateway using ICMPv6 Router Solicitation and Router Advertisement messages.
So if no better explanation comes along, you can consider IRDP to be an attempt to implement what we nowadays have in IPv6. It probably came too late to IPv4 to be successful, however, in IPv6, it is an inseparable part of the protocol.
Best regards,
Peter
