IPv4 has no equivalent to RAs, so the workaround is to infer prefix assignment
from address assignment.
Technically that's not quite correct:
RFC-950 defines a subnet mask request/reply.
And RFC 1256 defines router advertisements
Subnet mask ICMPs were never popular. I think some popular implementation
tended to give a wrong reply.
Router advertisements were somewhat popular. But got out of fashion when
DHCP became the standard way of obtaining configuration.