As the points out, Android 12 adds a toggle to disable the 2G modem. 2G is insecure and vulnerable to interception, so you're advised to avoid it.
eff.org/deeplinks/2022
Unfortunately, this feature requires version 1.6 of the Radio HAL, and many devices don't have that.
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GrapheneOS has supported LTE only mode for years. It's something we consider an attack surface reduction feature. Avoiding 2G/3G doesn't make cellular networks secure/trustworthy. It's important to use authenticated encryption and avoid trusting networks.
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Most phones including the stock Pixel OS allow carriers to disable user configuration of the network mode and the new 2G toggle.
GrapheneOS uses the stock Pixel OS APN / CarrierConfig databases as the input to generate ours but we strip the configuration disabling user control.
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LTE only mode option is quite usable now that VoLTE is very broadly deployed and supported. If you don't have VoLTE, you can't make/receive carrier-based calls. Carrier-based calls and texts should only ever be used as a last resort if you care about privacy/security regardless.