Hi, I'm John! I will help you with this.
Unfortunately, there isn’t a built-in way to change it back to the old envelope style.
What I’d suggest is sharing this feedback directly with Microsoft through the Feedback Hub app (Windows key + F key) or through Outlook’s built-in feedback option. https://support.microsoft.com/windows/send-feedback-to-microsoft-with-the-feedback-hub-app-f59187f8-8739-22d6-ba93-f66612949332 That way, your voice gets to the product team, and if enough users feel the same, it could influence future updates.
I know it’s not the same as having the choice right now, but your input is valuable and could help bring back a more visible icon in future releases.
Let me know if you have any further questions or concerns.