The RFC that explains the attribute and how it functions: https://tools.ietf.org/html/rfc4818
The page that shows that support was added https://wiki.mikrotik.com/wiki/Manual:RADIUS_Client#Access-Accept
And here is a document explaining how to use the feature with DHCPv6 in general (non-PPPoE): https://wiki.mikrotik.com/wiki/Manual:IPv6/DHCP_Server#RADIUS_Support
Unfortunately, it looks like MikroTik hasn’t yet created a detailed how-to on their wiki yet for this attribute with PPPoE.
The feature may be valuable for Sonar users wishing to deploy IPv6 even without PPPoE, in just a regular DHCP environment, since the RADIUS-controlled dual stack queue feature would allow for a single dynamic simple queue to rate limit both IPv4 and IPv6 for a customer in one queue. The current way you do the queueing with PCQ and address lists, each customer will get their own separate rate limit for IPv4 and IPv6, which is obviously not ideal when they are just buying one package.
We haven’t tested 6.43.x yet since it is brand new, and our firmware policies will prevent us from upgrading for a few months at least, but once we do, we would like to begin making use of this attribute almost immediately, so it would be great if Sonar had support.
I believe MikroTik is also currently working on IPv6 RADIUS accounting, but that function isn’t there yet to my knowledge.