I'm not sure what the best current practices are for mobile, but for residential/business/etc., the most convenient behavior would be for the delegated prefix to be static unless it is requested to be changed. That tends to be the case already with ISPs who grant public IPv4 to customers - it's DHCP whose lease stays the same unless you forcibly change your MAC address or let the lease expire via turning off your modem, and so on.

Talking about IPv4 with NAT, a "consumer" with zero server-hosting needs could get away with a changing public IP. Someone with any kind of server needs, like hosting their own Internet-accessible IoT portal, personal VPN, website, game server, etc. would want a stable public IP address.

Yeah, that may lead to tracking, but it's the status quo, I suppose is the point.

In case it isn't known, IPv6 has so many addresses and is designed in such a way, that it is expected each "network" (think home network) would be given a network prefix of 56 or 60 bits. The "host" portion of an IPv6 address is the final 64 bits of the address. Therefore, each network an ISP issues to a client should have room for something between 16 and 256 subnetworks, each with effectively unlimited client address space.