What is the Kaspa testnet release and should I download it?

No — the testnet release is a developer testing environment, not the live Kaspa network you need for real KAS. When browsing the official release page you will notice some entries have the word 'Testnet' in their name. These connect to a sandboxed network used by developers to experiment without risking real funds. The correct download has no 'Testnet' postfix in its release name. This distinction matters because installing the testnet version means your wallet will talk to a network where nothing has real value, and you will not see or be able to use your actual KAS.