Google disagrees - https://itsfoss.com/goobuntu-glinux-google/

Debian security team used to offer support for testing, but as the distro grew and attack surface increased, that was dropped. However, security incidents are patched in unstable as well and they float up to testing relatively quickly (for a desktop system, anyway). When things freeze, security updates directly land on testing as a fixup polish.

However, while Testing is "relatively secure", it's never "Server secure". I'd never use testing or testing packages in stable or any server for that matter.

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