Following some discussion at RIPE84Fredy Kuenzler and I put forward an Internet Draft to define a well-known advisory BGP community to denote prefixes are used for Anycast.
To quote from the abstract of the ID:
> In theory routing decisions on the Internet and by extension within ISP networks should always use hot-potato routing to reach any given destination. In reality operators sometimes choose to not use the
hot-potato paths to forward traffic due to a variety of reasons, mostly motivated by traffic engineering considerations. For prefixes carrying anycast traffic in virtually all situations it is advisable to stick to the hot-potato principle. As operators mostly don't know which prefixes are carrying unicast or anycast traffic, they can't differentiate between them in their routing policies.
> To allow operators to take well informed decisions on which prefixes are carrying anycast traffic this document proposes a well-known BGP community to denote this property.
You can find the lastest version of the draft at: https://datatracker.ietf.org/doc/draft-wilhelm-grow-anycast-community/