Theoretically this ability should remain active regardless of using a UK Apple ID, as it's tied to the device's region
But what makes most people have to switch eventually is a bank/government app that is region locked and you need it to live your life without pulling your hair out jumping through stupid hoops.
A VPN and two devices is the only way to stay truly separate from the region you live in full time over the long term.
I have been using an iPhone with a British Apple ID and the British App Store, and the UK regional settings on some devices despite not living there anymore for a couple of years now. I am fairly confident switching countries requires the user to do it consciously.
[edit] for this feature specifically, they are more aggressive: https://www.theverge.com/2024/3/7/24093437/apple-iphone-thir...
I don’t think they change anything about your accounts or device settings themselves, though.
So you can have a single iPhone with both US and UK banking apps. Same for Pixel.
The apple ID used for the app store doesn't have to be the same as the apple ID for Facetime, Messages etc. You don't even need to be signed in to the app store to use an app that's already installed.