From the RFC:
One of the biggest challenges of deploying IPv6-only LANs is that such networks might contain a rather heterogeneous collection of hosts. While some hosts are capable of operating in IPv6-only mode (either because the OS and all applications are IPv6-only capable or because the host has some form of 464XLAT [RFC6877] deployed), others might still have IPv4 dependencies and need IPv4 addresses to operate properly. To incrementally roll out IPv6-only, network operators might need to provide IPv4 on demand, whereby a host receives an IPv4 address if it needs it, while IPv6-only-capable hosts (such as modern mobile devices) are not allocated IPv4 addresses.
ISC-DHCP provides
option v6-only-preferred uint32; The number of seconds the client should disable DHCPv4 for. This option and its use by the client are specified in RFC 8925.