Because Eaglercraft exists in a legal gray area (it requires assets from Mojang/Microsoft to function), you won't find it on the official Microsoft store. However, the open-source community maintains several stable mirrors.
: It is compatible with ChromeOS, Windows, macOS, and mobile browsers. Server Compatibility eaglercraft 1.9 launcher
Warning: Your saved singleplayer worlds will not transfer between launcher versions without manually extracting the level.dat from IndexedDB. Because Eaglercraft exists in a legal gray area