I vaguely remember Vitalik mentioning multiproofs in the context of stateless clients for Ethereum 2.0, but this is what I could find: https://ethresear.ch/t/open-research-questions-for-phases-0-... and the code: https://github.com/ethereum/research/tree/master/merkle_tree

I also found a good article introducing multiproofs from Jim McDonald here: https://www.wealdtech.com/articles/understanding-sparse-merk...