Here are some useful information on PHP 7.4 update.
Please check if there will be any impact on your sites.
There are some things that are going away, so any rules that are added to .htaccess will no longer work.
You can check this article here https://wpengine.com/support/htaccess-deprecation/ Other than that there isn’t much other changes, you can find here https://www.php.net/ChangeLog-7.php
As of right now it’s not a mandatory update, But you can try it out on staging or dev site to see its impact.
https://wpengine.com/blog/a-first-look-at-php-7-4/ Here is more user friendly representation regarding changes in 7.4. Look at the “Deprecations” and “Performance Enhancements” sections.
*** If your site has redirects in the .htaccess, they will need to be relocated in order to remain both functional and optimized 🙁