I would use PHP 8.1 in a production environment, but as there's only 7.4 available…
But I never had only a production environment - every live system has its development environment, and there I want to read warnings, deprecation notices etc.; So of course I want to install the newest PHP version on my local machines. Always. Just to be prepared for future server updates. Reading changelogs is of course mandatory (PHP offers great documents for upgrading your code).
I never had any problem to get my web-apps / websites and scripts up to the newest PHP version while running the same code on the live system with older PHP versions for many, many years now, and it never costs me much time to update my code (the only big one was the change from the old MySQL extension to the actual MySQLi - must been around 2015; creepy stuff like "register_globals" I've never used - even in times other did (but I had to deal with such code)). Already fixed a really huge project to be 8.1 ready
But if you're using third party PHP code: In this case I would check the informations provided by the developers - they should know if PHP 8.1 is usable or not.