The AICPA has published some very useful guidelines:
https://www.cpai.com/Education-Resources/my-firm/Tax-Services/What-accounting-firms-need-to-know-about-CTA?refID=
Two key points:
"To help reduce the likelihood of claims asserting a failure to advise a client of CTA, consider sending a newsletter or other general client notification letter to all clients informing them of CTA and its reporting requirements. Retain a copy of the newsletter as well as the distribution list. "
"Unless specifically engaged to provide CTA assistance under a separate engagement letter, include a provision in all engagement letters, regardless of service, disclaiming a responsibility to do so. A sample provision is provided in this Risk Alert."
After reading this, I am going to draft a separate letter notifying my clients of the CTA and the reporting requirements
declining to provide any services or advice related to the CTA and it's reporting requirements
which I will require my clients to sign in addition to my annual tax authorization/engagement letter.