LONDON—Stablecoins would have to comply with the same safeguards as their more traditional competitors in payments under proposals from regulators on Wednesday as authorities get to grips with a rapidly evolving sector. Stablecoins are cryptocurrencies designed to have a stable value relative to traditional currencies, or to a commodity such as gold, to avoid the volatility that makes bitcoin and other digital tokens impractical for most commerce. Facebook Inc.’s move in 2019 to introduce its own stablecoin Diem, then known as Libra, raised concerns among governments and central banks that a major payments competitor could emerge overnight with little regulation. Since then, Diem has radically scaled back its ambitions and plans to launch a U.S. dollar stablecoin. The IOSCO group of securities regulators and the Bank for International Settlements, a global forum for central banks, set out on Wednesday how current rules for major clearing, settlement, and payments services should …