You can now set Outlook.com as your default mail client for mailto: links.
The feature is being advertised on Outlook.com as a banner and requires a compliant browser.
The feature uses the “URL protocol handler registration for PWAs” functionality, a feature that rolled out with Chrome 92.
To change it back you would need to visit the Default App settings in Windows 10 or 11.
The feature is rolling out in a staged manner and may not be available to everyone right away.