I just checked on my GTMs, running version 11.2.0, i dont have Rate Shaping in the UI, i also dont have Acceleration! Maybe you shuld ask Support.
also, am not convinced the rate shaping of TXT and big DNS responses is all that useful... I would imagine reflection and amplification attacks would use your DNS server, among many other thousands of them, to generate the DDOS traffic that they need to overwhelm their victim, which isn't your DNS server.. They would slow down they queries to you to the minimum they could get away with, in the hope that you won't even notice it.... anyways... I tried to implement a response rate limiting.. I'll share if you are interested.