These are relatively common in theory and practice.
- uniswap v2 oracles: https://uniswap.org/docs/v2/core-concepts/oracles/
- chainlink: https://docs.chain.link/docs/using-chainlink-reference-contr...
- schelling point oracles: https://blog.ethereum.org/2014/03/28/schellingcoin-a-minimal...