Basic spellcheck support has been in place for quite a while now.
Grammerly will require a good bit more work and Coda is still growing quite a bit at the moment. We do hear you that this is a wanted feature for sure, we’re just working on building out the product more before committing to the effort investment here.
When growing a product you find that adding features means there is more to account for, and work around, when adding larger changes to the foundational items. It’s easy to build features to the point of hindering future growth too. So our team puts a lot of thought into how they juggle what gets added and when. The order can have a big effect down the line. Efficient ordering of procedures doesn’t always correlate to order of importance.
We have this noted and it is discussed.