This class was added to resolve T37247: content-holding <div> should only contain the page text to fix pending issues with TemplateStyles. For the styles added by TemplateStyles to be applied properly in Flow comments, those comments should be enclosed in an element with class mw-parser-output. Other parts of the Flow page should not be enclosed in this class.
If nothing else, chances are you can add it to the same div that already has class flow-post-content.
This should also be verified to be true when creating a Flow post; T164790: Add class mw-parser-output to VE's editing area may be relevant to that.
In addition to posts, should also include header/description and summary.