Keep in mind that this question does not include all users.
- I am a screen reader:
I do not care about the form of an iFrame, I am the instructions of this DOM object.
- I am a layer of behavior:
I do not care what is indexed, I am improving what is here on this DOM object.
- I am a disabled user and use screen readers, voice command software and behavior layers to understand / interact with web content:
I do not care what is indexed and interact with current information regardless of presentation layers.
- I have full visual and articulated faculties and I want a "good" UX:
There are so many UX websites and options, but I can find some trends. With so many options, the best security system I can use is based on the intention of the ecosystem I am currently in. What did the brand learn? What other layouts have taught me? Does this current (if not global) interaction compare to other site interactions? In line with my expectations? When I interact with this iFrame, is its presentation unexpected compared to my current environment?
At least that would be defensible from the point of view of the brand, even the UX.
The goal is to look for a defensible job, right? If you ticked all the boxes up to 99, then I think your measurement criteria will more than likely comply with your trademark guidelines (or at least such as they dictate the UX ) there are still questions.
This does not mean that your brand can not be something unusable if …
* Note: Opening this answer is not supposed to be hard, but rather giving a starting point to indicate the transition view in defensible arguments that we can use to work 🙂