> Where did you see that sentence "[...]should never be present in a non-enhanced DICOM IODs" ?
It doesn't appear anywhere in the standard. So I must prefix it with "In my opinion."
But here is the justification. In order to be meaningful, the SharedFunctionGroupsSequence tree will use at least some the attributes from the enhanced functional group modules. But in legacy IODs, those same attributes (or their equivalents) are stored in the root of the data set. So the use of SharedFunctionGroupsSequence or PerFrameFunctionGroupsSequence in an extended legacy IOD will lead to duplication of information and difficulties with interpretation.
So, no, it's not disallowed by the standard (as far as I know), but it's a bad idea and I hope no vendors do such a thing.
In a recent discussion, you argued that no MR data set could have a GE PDB and a Siemens CSA. I agree, because, there would be huge problems interpreting such a data set, and no practical reason for anyone to generate such a data set. But can you point to a part of the standard that says it's not allowed?