3G devices should still work over 2G. It's much slower, but it works and should do so until well into 2030 in the UK.

The problem with 3G as I understand it is that it uses more power and is less efficient than 4G/5G. They're starting to re-deploy the 3G bands as 4G/5G, so the other Gs will eventually benefit from this shutdown.

The lower-bandwidth connections get completely saturated by modern phones with modern data allowances. Back in the day I had 500MB a month on 3G, for instance. I can use that in a few minutes these days.