The RFC on such interoperability (1534) is clear: “In summary, a DHCP SERVER:

MAY support BOOTP clients,” (SECTION 2). The word “MAY” indicates such support, however useful, is left as an option. A source of CONFUSION on this point is the following statement in section 1.5 of RFC 1541: “DHCP must provide SERVICE to existing BOOTP clients.”
However, this statement is one in a list of “general design goals for DHCP”, i.e. what the designers of the DHCP protocol set as their own goals. It is not in a list of requirements for DHCP servers.

The RFC on such interoperability (1534) is clear: “In summary, a DHCP server:

MAY support BOOTP clients,” (section 2). The word “MAY” indicates such support, however useful, is left as an option. A source of confusion on this point is the following statement in section 1.5 of RFC 1541: “DHCP must provide service to existing BOOTP clients.”
However, this statement is one in a list of “general design goals for DHCP”, i.e. what the designers of the DHCP protocol set as their own goals. It is not in a list of requirements for DHCP servers.