Last Friday, Microsoft released Windows 10 build 18963 to Insiders in the Fast Ring, and with it came the announcement that Notepad, one of the most notable system apps that's shipped with Windows for years, would be updateable through the Microsoft Store in the future. Now, as spotted by Reddit user Leopeva64-2, the Store listing for Notepad is now live.
The listing can be accessed on current versions of Windows 10, but installing the app won't actually work unless you're in the Fast Ring of the Insider program, in which case the listing will merge with the system-bundled version of the app. On older builds, installing the app will create a separate entry in the Start menu, and it won't be usable.
For now, there's not a whole lot that's changed when you install the app from the Store, as this is just the initial release to make the app independent from Windows updates. Going forward, it should enable Microsoft to deliver new features and improvements more quickly than what's been possible with Windows 10's semi-annual update cycle.
If you're interested, you can download the Notepad app from the Microsoft Store.