Bitcoin Core 28.0 added support for Testnet4 as specified in BIP94. The release introduced a testnet4 option and a matching configuration section while retaining Testnet3 support in that version.
The same notes describe stricter JSON-RPC 2.0 handling and a change to the default data directory on Windows. Existing installations with the older directory were intended to continue using it.
This is an archive brief about the 28.0 release. The series has since reached end of life, as documented in the 31.0 notes. Test-network support helps developers work in a separate environment; it does not turn a test setup into the main Bitcoin network.