Excerpt from RFC6204 / RFC7084:

Basic Requirements for IPv6 Customer Edge Routers
4. Requirements
4.1.  General Requirements
G-5:  By default, if the IPv6 CE router is an advertising router and
        loses its IPv6 default router(s) and/or detects loss of
        connectivity on the WAN interface, it MUST explicitly
        invalidate itself as an IPv6 default router on each of its
        advertising interfaces by immediately transmitting one or more
        Router Advertisement messages with the "Router Lifetime" field
        set to zero [RFC4861].

4.3. LAN-Side Configuration
L-13:  If the delegated prefix changes, i.e., the current prefix is
         replaced with a new prefix without any overlapping time
         period, then the IPv6 CE router MUST immediately advertise the
         old prefix with a Preferred Lifetime of zero and a Valid
         Lifetime of either a) zero or b) the lower of the current
         Valid Lifetime and two hours (which must be decremented in
         real time) in a Router Advertisement message as described in
         Section 5.5.3, (e) of [RFC4862].

Especially Windows clients can't handle dead IPv6 prefixes which still have any valid or preferred lifetime left. I know, I can use a script, but I don't like scripts.

Yes.

This needs to be fixed, e.g. FreeBSD and MacOS apparently need this behavior in order to deal with changing prefixes correctly.

I don't know if it is allowed, but I'd like to bump this request.
I know it isn't the priority of the team, but don't forget it, please!

Thanks in advance

+1 for this

I would like to join the request for RFC 7084 to be supported.
On residential connections that the wan can be unstable, I had to either set ridiculously short lifetimes on prefixes (it works, its sub-optimal/messy),
or I end up with ghost prefixes assigned on hosts.
For now I just disable ipv6 altogether.
I suppose/hope that with the v. 7 using a much newer kernel the support should be already there,
and so implementing the correct behavior should be easy.