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.
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.
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Samsung One UI also has the 'LTE only' mode. Probably borrowed from you guys.
LTE only mode is good, but extremely rarely my phone switches to HSPA when there is bad reception.
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That means you aren't using LTE only mode. LTE mode is not LTE only mode. You can verify you're using LTE only mode by disabling VoLTE and testing a call. If you can make a call, you aren't using LTE only mode.
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This is good. what a pity not even all carriers support the VoLTE even in 2022. So turn on this feature will broken the calls and emergency.
The Samsung also allow it when using the Band Lock. I used to lock just for LTE cuz even with low signal, the 4G is faster than 3GinBrazil