I don't think this setting does what you think it does. The description under this option has a big caveat: "For emergency calls, 2G is always allowed". So even when disabled, the phone can still use 2G networks.
It sounds like this new option is to actually disable all 2G functionality.
Previously (e.g. Android 13) if your cellular provider said you can choose "No" then there's a slider in your Settings so you can choose, otherwise it's greyed out and tells you "Name of Your Provider requires 2G to be available".
On corporate devices it's a pain to have settings that your users can screw up, and so it's nice when Android (or Apple for iOS) makes the setting something you can choose to lock down instead.
# Edited to correct text of fixed message when your cellular provider has decided you mustn't switch off 2G.
There is also nothing the OS can do to shut down 2G in most cases; the best it can do is ask nicely that the baseband does not use it. So the "shut it all down" theory is also not correct.
It’s probably a legal requirement that if 2G is supported emergency calls on 2G must be also supported. Maybe there are some LTE-3G dualmode phones without GSM in the world, if that’s what you must have?