The testnet ran from August 2019 to January 2022. Some incompatible tests were performed, which required special rules in the program code. A testnet should also be as close as possible to the mainnet. For the switch to deterministic masternodes in 2022, a clean testnet is useful before the productive mainnet follows.
On January 8, testnet4 was launched and the previous testnet3 was archived. Both blockchains can be accessed with the Explorer on dms.cash.