Codeigniter is work 100% with PHP7.4.3? 07/03/2020

A framework is a productivity tool, its main objective is to make tiring and repetitive tasks transparent to the developer. In fact, it creates bridges so that we can use our brain in a more noble function: thinking.

Any incompatibility with PHP must be a problem with your code, with each new version, when they exist, deprecated functions are reported.

Do not worry about CI, otherwise you are going against the reason it exists.