I was at the RFC meeting today, and we discussed https://phabricator.wikimedia.org/T89733 .
I don't think it necessarily affects Flow, but it might, especially if we do the "a Topic is one document" thing we keep talking about on-and-off.
Basically, there is currently a Content::getTextForSearchIndex(), but this doesn't support structured data.
The proposal is to define various methods for Content and ContentHandler, which would allow structured field definitions. For example, we might have a originalPoster, commenter (fields can be multi-value), etc.
The search backend stuff is pretty much done AFAIK, so this is more of a note for the future.
Matt Flaschen