ISC DHCP server config option limited-broadcast-address
There is no manual entry for this. The idea is to get some control on the broadcast address used to send responses to on-link clients. A priori not a bad idea. BTW the limited broadcast address is 255.255.255.255 and is not forwarded by routers (so the name) but the ISC DHCP option takes an IPv4 address (vs a boolean).