Based on feedback here and questions from various users, most people don't notice the "Existing wikitext discussions are moved to an archive: "
Enables a new structured discussion system on your user talk page. Flow simplifies talk page discussions with clear places to write and reply, and allows conversation-level notifications. Existing wikitext discussions are moved to an archive. This feature is not auto-enabled; users will have to enable it separately. Disabling this feature will move the Flow board to a subpage and un-archive the previous talkpage.