I know DHCP can be used to inform hosts about routers on the network, so this ICMP alternative isn't strictly necessary.
But I found RFC 1812: Requirements for IP Version 4 Routers, which states:
4.3.3.10 Router Advertisement and Solicitations
An IP router MUST support the router part of the ICMP Router
Discovery Protocol [INTERNET:13] on all connected networks on which
the router supports either IP multicast or IP broadcast addressing.
The implementation MUST include all the configuration variables
specified for routers, with the specified defaults.
The “INTERNET:13” reference is RFC 1256: ICMP Router Discovery Messages
Is ICMP Router Discovery supported in LEDE/OpenWrt?
I cannot find any mention in dnsmasq or odhcpd.
P.S. I know that in IPv6, Router Discovery is part of the Neighbor Discovery Protocol, and I know this is well supported by odhcpd. I'm interested in IPv4.