Hello,
I believe this feature was introduced by Cisco and there is no RFC for that at this time I am writing this. There are many protocols and features which do not have any RFCs. For example, EIGRP has been around for a long time, but Cisco has released it as an RFC recently.
Check the RFC for EIGRP. It was released in 2016.
https://tools.ietf.org/html/rfc7868