Is Grammarly going to be allowed as an extension in Pages?
I use both Pages and MS Word. MS Word allows an extension for Grammarly. This would be a valuable addition to Pages.
Pages is neither a Word clone nor provides extension support. As the Pages product team is solely aware of any future Pages product planning, and they do not participate here, future-oriented questions will go unanswered.
Grammarly does offer a Mac application download from their website, and you might try that with Pages to see if it cooperates. This application requires that you are signed in to a Grammarly account before it will function.