EIP specifying a testnet mechanism for client implementations that automatically periodically resets the network back to genesis. This kind of testnet can provide an alternative ephemeral environment to long running testnets or devnets.
PR with the EIP doc: Add EIP: Automatically Reset Tesnet by taxmeifyoucan · Pull Request #6916 · ethereum/EIPs · GitHub
EIP is based on previous work on existing network Ephemery. The existing network can be used as a custom network in any Ethereum client and has been running for months now.