There are several projects that are willing to offer Flow as opt-in on user talk pages -- Catalan, French, Hebrew, possibly Wikidata.
Switching Flow ON for your user talk page would be the same as using Special:EnableFlow. The wikitext page is moved to /Archive 1+, and the Flow board is created in its place.
Switching Flow OFF would move the Flow board to /Archive Flow 1, and the most recent wikitext page would move back.
Going back and forth, the wikitext archive and Flow archive are swapped -- we're not creating new Flow boards every time.
See Pau's prototype for the model.
Beta features page
Heading: Flow on user talk
Text: 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. Disabling this feature will move the Flow page to a subpage, and un-archive the previous talkpage. This feature is not auto-enabled; users will have to enable it separately.
On each wiki, the "information" link should point to that wiki's version of Project:Flow, and "discussion" to Project_talk:Flow.
Clicking the Save button on the Beta features page triggers the Flow board creation and wikitext archive.
This feature will NOT be automatically enabled, even if the user has "Automatically enable all new beta features" checked.
Notification
When the user presses save, they get a notification, as seen in the mockup:
New discussion system enabled
on your user talk page
Available at User talk:Username
The notification links to the user talk page.
On the user talk page, the user will see a three-step modal "guided tour"; that spec is in T108266: Opt-in: Guided tour on user talk for first visit to new Flow board
Templates with archive links
When a wikitext page converts to Flow, we add a template on two pages:
On the archived wikitext page, we add {{Archive for converted wikitext talk page}} at the top
On the new Flow page, we add {{Wikitext talk page converted to Flow}} in the side rail
When a user disables Flow and moves back to wikitext, we should remove those templates automatically.
If somebody has edited either page in a way that makes the template unrecognizable, then it won't be removed. The person who edited the page can edit it again, if they want to.
Text for email notification
Title: New discussion system on User talk:Name
Text:
Flow, the new wiki discussion system, has been enabled on your user talk page on {{SITENAME}}.
You can get more information, provide feedback or disable the new system any time from the Beta features section in your preferences.
Button text: View user talk page