Turning off the chat is a great feature.

Unfortunately, it has a few small problems.

a) it's hard to find - but that's the smallest issue
b) you can't turn off the chat for only one game (which I would deem the default behavior)
c) if you have turned off the chat (for instance, a few games back, and forgot about it), there is no feedback when using the chat that it is turned off - this makes you wonder why there is no response to your chat - until you realize the situation eventually
d) if you use the chat, there is no feedback that the other person has muted you - same result as in c, but uncheckable

Thus, I would suggest that if the chat is turned off by at least one side, the chat-writing feature should also be disabled. That way, it will become apparent to the person wanting to chat that either they have it still turned off or the other person won't read what you write anyway.