Lockdown mode is iOS 16.
The press release [1] doesn't mention 2G directly but only "safer wireless connectivity defaults" but FastCo [2] is more direct "with iOS 17, Apple is not only beefing up Lockdown Mode (by blocking the iPhone from connecting to 2G cellular networks and from auto-joining insecure wireless networks) but bringing Lockdown Mode to the Apple Watch".
[1]: https://www.apple.com/newsroom/2023/06/apple-announces-power... [2]: https://www.fastcompany.com/90904197/apple-ios-17-craig-fede...