Can Microsoft see all Windows notifications?

overuse:

is there a way to know if a windows app uses WNS

Probably, but it’s cumbersome.

However, when browsing through event viewer, I noticed there is a policy in gpedit.msc to disable WNS (do so at your own risk) under Computer Configuration > Administrative Templates > Start Menu and Taskbar > Notifications > Turn off notification network usage:

This policy setting blocks applications from using the network to send notifications to update tiles, tile badges, toast, or raw notifications. This policy setting turns off the connection between Windows and the Windows Push Notification Service (WNS). This policy setting also stops applications from being able to poll application services to update tiles.

If you enable this policy setting, applications and system features will not be able receive notifications from the network from WNS or via notification polling APIs.