GUSD backing, also audited by a large US accounting firm, subject to New York State Department of Financial Services regulations: https://www.gemini.com/cryptopedia/gusd-stablecoin-gemini-do...
USDP backing, also audited by a large US accounting firm and subject to New York State Department of Financial Services regulations: https://paxos.com/usdp/
DAI backing, viewable transparently on-chain. Currently backed 168% by collateral: https://daistats.com/#/overview
LUSD backing, viewable transparently on-chain. Currently backed 189% by collateral: https://dune.com/dani/Liquity
UST (Terra/Luna) and USDT (Tether) have/had nothing like this.