I was recently working with an iPhone user who noticed that since updating to iOS 14, they were being prompted to set up two-factor authentication on their Apple ID. They temporarily dismissed the prompt by navigating to Settings, tapping the "Two-Factor Authentication" option and selecting "Not Now" - but out of interest, will this prompt re-appear - or does choosing this option disable all future prompts from appearing on that device (or Apple ID)?
You cannot guarantee that the user will not be prompted in the future.
There's a continued development in security and the need for preventive measures. Today two-factor authentication is absolutely crucial and wide-spread, 25 years ago it was only used by the few.
Apple some times "moves the bar" by changing the minimum security requirements. In those cases, they could start prompting user's again for enabling two-factor authentication - or even requiring it entirely.
For example, not so long ago the requirements for developer accounts were changed so that these users must have two-factor authentication enabled (also affects apps such as TestFlight, App Store Connect, etc.). These users were notified in advance that this requirement would be instated.