On a positive side, lack of this feature may have kept many people safe from this soon to be disclosed vulnerability:
http://forum.mikrotik.com/t/uknof-43-cve/128988/1
http://forum.mikrotik.com/t/running-ipv6-on-mikrotik-youre-out-of-business-in-12-days-time/129015/19
Stop IPv6 deployment, or move from MT to something else (Cisco, Linux + BIRD + accel-ppp, …) so you can deploy IPv6 anyway = not vulnerable 
Since it was known internally for about a year, perhaps it was the real reason support was not telling anyone why it takes so long to implement Delegated-IPv6-Prefix.