The new Outlook will be installed automatically on Windows 10 in an upcoming update. You cannot block the app from being put on your PC, though you can remove it after installation.

Microsoft will force install the new Outlook email client on Windows 10 systems starting with next month's security update.

The good news is that the February update which brings the new Outlook doesn't strip out the old version of the app, so you can still use that.

Users of doomed operating system to receive unloved app via an update Microsoft's new Outlook will be automatically installed on Windows 10 machines as part of an upcoming monthly security release.… According to a message in the Microsoft 365 message center,

The new Outlook app will be packaged in an optional download for Windows 10 starting January 28th, but after the February 11th security update, it will be be a forced inclusion. The new Outlook app will work alongside the old one without changing any current settings or configurations.