There have been several requests to add support for a grammar checking tool like Grammarly or LanguageTool (the latter is open source).
If we were to implement this in any fashion, we’d have to go with LanguageTool, which has a public API. If implemented as a core feature, this would need to be a Pro feature, since the API is not free. (There is a free version of the API, but it may be too limited to be practical.)
Grammarly does not have a public API.
This may alternatively be a good candidate for a plugin.