Respect the Windows 10 global spellcheck setting
Windows 10 allows you to disable spellcheck globally. This is important to customers who write in more than one language. Almost no UWP apps from Microsoft respect this setting (OneNote, for example, ignores this as well).
Spellcheck and it's annoying litte red worms are real frustration. Why do you assume that we all want spellcheck everywhere?
Stephen Ryan commented
Spellcheck can be disabled. Rename the folder in your profile containing the component,
You willneed to repeat this each time the app is updated.