Current eth1.x work is putting this approach out of scope right now as it adds significant complexity to the incentives of the system. Remaining Questions on State Providers and Stateless Networks in Eth2 - #8 by burdges talks about this a little, and another writeup is coming that will state additional challenges (particularly on how the access-list approach has a few caveats). Also, the relayer (instead of state provider) approach can skew towards a number of issues - briefly described in this post and comment. Relayer has typically been used to describe an actor that packages the full block for the block producer vs. prepare witnesses for a user (state provider) which I think you are referring to.