"Will MATLAB Live editor ever support spellcheck?"
You should assume that the answer to all questions about whether MATLAB will ever support a particular feature, is NO.
There are an infinite number of features to be implemented, and a finite amount of resources and time to implement them, so the statistical probability is a finite number divided by infinity, which is Zero.
In extremely limited cases, some Mathworks insiders have knowledge of features that are scheduled to be released within a few months. However, Only the product managers and upwards are authorized to release information about forthcoming releases. Oh, and Sales.
If the matter is important to you, ask your Sales person, who might be able to get some response under Non-disclosure Agreement. But the people who read your question here either don't know or cannot tell you what they know.