On first glance, I am not sure what we are permitted to do about this. The relevant IETF specifications forbid us from using non-temp IPv6 when temp IPv6 are available. If the VPN wants to disable temp IPv6, it probably needs to _remove_ those addresses entirely, instead of simply making them drop packets.
Bug 1661059 Comment 4 Edit History
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On first glance, I am not sure what we are permitted to do about this. The relevant IETF specifications forbid us from even disclosing (much less using) non-temp IPv6 when temp IPv6 are available. If the VPN wants to disable temp IPv6, it probably needs to _remove_ those addresses entirely, instead of simply making them drop packets.