A donor-advised fund devoted to supporting the United Nations Sustainable Development Goals has leapt from being a relatively minor charity to one with an asset size comparable to behemoths like the Andrew W. Mellon and David and Lucile Packard foundations. The SDG Impact Fund, based in Cartersville, Georgia, grew from $238 million in assets in 2020 to $10 billion in 2021. That eye-popping growth, which seems to have been fueled by the meteoric rise of cryptocurrencies and digital art assets, has prompted some questions from philanthropy and tax experts. The less stringent legal reporting requirements for DAFs compared with private...