Separate Trust Document - Not a Requirement?

Is there any reason why a plan with individual named trustees would be exempt from having a separate trustee agreement that includes provisions covering such matters as the powers and duties of trustees, investment authority, and the kinds of investments that may be made? 

Here's some background, a client of ours is changing custodial platforms and the custodian has requested a copy of the plan's trust agreement. This is very standard in my experience. I noticed we didn't have a copy of the signed trust agreement for Cycle 3 and asked the client for a copy. The advisor reached out and indicated that they thought a plan with an individual trustee wasn't really required to have a trustee agreement. The plan uses a pre-approved document and the document provider does provide a standardized trust agreement. 

I know that they are not required to use the document providers standardized trust agreement but I've never heard of a plan being exempt from having one. 

Me neither. If I may add a follow-on question: can a plan sponsor even have a trust without a trust agreement? Finally, in the absence of insurance and a custodial account, I've always thought a qualified plan needs to hold its assets in a trust account. True?

Even if the retirement plan is not ERISA-governed and a relevant State’s law recognizes an oral trust, the IRS might assert that a written trust is a condition of I.R.C. § 401(a)-qualified tax treatment.

Consider, for example, 26 C.F.R. § 1.401-2(a)(1) https://www.ecfr.gov/current/title-26/part-1/section-1.401-2#p-1.401-2(a)(1).

That the Treasury’s interpretation speaks of what the trust instrument must provide suggests the Treasury’s interpretation that the trust must be written.

While there might be other interpretations, few clients want unnecessarily to interpret tax law contrary to a long-established mainstream.

This is not advice to anyone.

Thank you, this makes sense to me.