Dear Mathieu Anfosso,
Good day! Thank you for posting to Microsoft Community. We are happy to assist you.
As per the description you have shared, we understand that you have a concern with missing Conditional Formatting feature in the New Outlook.
We do understand your concern here and we do apologize not to meet your requirements. Yes, you are definitely right, Conditional formatting is a way to make messages that meet defined conditions stand out in the message list by using color, fonts, and styles in the classic Outlook. However, in the new Outlook, conditional formatting doesn't seem feasible at this moment. Therefore, thank you for trying New Outlook for Windows and your valuable suggestion. At this point, since New Outlook is still under development, we will surely raise your concern to our Product Development Team and hopefully this can be included in the future updates. In the meantime, we sincerely recommend you post your valuable suggestion as an idea on our in-app New Outlook feedback site. To provide feedback, go to Help> Feedback. You can refer to Contact support and provide feedback within the new Outlook for Windows - Microsoft Support

Our Product Development Team are committed to enhancing the customer experience by continually examining suggestions to include in the design and development of future product upgrades based on your suggestions. Hopefully your suggestion will help many users in the future.
Thank you for your precious time, valuable suggestion and understanding. For other concerns, please do not hesitate to add your post in the Microsoft Community Team.
Stay safe and healthy. Have a nice day!
Sincerely,
Libeamlak | Microsoft Community Moderator