See https://phabricator.wikimedia.org/T98270#1614412 .

@Etonkovidova:

There are some problems with opting-out.

Straighforward scenario

Create a User talk page and enter some text
Then enable "Flow on user talk" in Preferences - Beta
User talk page is converted to Flow without any problem. The previous page content is archived at User_talk:username/Archive_1
Go back to Preferences - Beta and disable "Flow on user talk" . The User_talk page keeps displaying Flow board and all Flow board's functionality is there.

deployment-fluorine /a/mw-log/exception.log consistently displays the following error :

2015-09-07 17:45:44 deployment-mediawiki02 enwiki exception ERROR: [da2719b3] /wiki/Special:Preferences Flow\Exception\FlowException from line 88 of /srv/mediawiki/php-master/extensions/Flow/includes/BoardMover.php: Main discussion workflow for 127519 not found {"exception":"[Exception Flow\\Exception\\FlowException] (/srv/mediawiki/php-master/extensions/Flow/includes/BoardMover.php:88) Main discussion workflow for 127519 not found
[stacktrace]
#0 /srv/mediawiki/php-master/extensions/Flow/Hooks.php(1587): Flow\\BoardMover->prepareMove(integer, Title)
#1 /srv/mediawiki/php-master/includes/Hooks.php(204): FlowHooks::onTitleMove(Title, Title, User)
#2 /srv/mediawiki/php-master/includes/MovePage.php(231): Hooks::run(string, array)
#3 /srv/mediawiki/php-master/extensions/Flow/includes/Import/OptInController.php(137): MovePage->move(User, NULL, boolean)
#4 /srv/mediawiki/php-master/extensions/Flow/includes/Import/OptInController.php(112): Flow\\Import\\OptInController->movePage(Title, Title)
#5 /srv/mediawiki/php-master/extensions/Flow/includes/Import/OptInUpdate.php(50): Flow\\Import\\OptInController->disable(Title)
#6 /srv/mediawiki/php-master/includes/deferred/DeferredUpdates.php(119): Flow\\Import\\OptInUpdate->doUpdate()
#7 /srv/mediawiki/php-master/includes/MediaWiki.php(706): DeferredUpdates::doUpdates(string)
#8 /srv/mediawiki/php-master/includes/MediaWiki.php(526): MediaWiki->restInPeace(string)
#9 [internal function]: Closure$MediaWiki::doPostOutputShutdown()
#10 {main}
"}