This can work with the fiber + LTE example, and with the rat example, but does not cover the "modem crash" example. The ADSL modem does not know its old prefix,

This can work with the fiber + LTE example, and with the rat example, but does not cover the "modem crash" example. The ADSL modem does not know its old prefix, and thus cannot send zero-expiry-time announcements for it. Also consider a case where there is an ADSL modem (with the ISP giving out /56 via prefix delegation) and a home lab with virtual machines, that are behind a virtualization host, which grabs a subprefix (let's say a /64, separate from the main home LAN /64 prefix) for its VMs from the modem via DHCPv6. While there is indeed a mechanism for flash renumbering over SLAAC, which may work for devices in the home LAN, there is also a need to invalidate the subprefix delegated for virtual machines via DHCPv6 through the virtualization host. Last time I checked, this is not implemented anywhere.