While updating from 3.0.14 to 3.1.9 and following the upgrade guide https://www.phpbb.com/support/docs/en/3.1/ug/upgradeguide/upgrade3/, I started the DB upgrade from the CLI:
php ./bin/phpbbcli.php db:migrate --safe-mode

After some success lines I encountered a fatal error:

Updated data: \phpbb\db\migration\data\v310\profilefield_location; Time: 0.08 seconds

Updated data: \phpbb\db\migration\data\v310\profilefield_location; Time: 0.04 seconds

Updated scheme: \phpbb\db\migration\data\v310\soft_delete_mod_convert; Time: 0.00 seconds

PHP Fatal error:  Call to undefined function phpbb\truncate_string() in /PATH/forum2/phpbb/content_visibility.php on line 424

This is solved by adding
require($phpbb_root_path . 'includes/functions_content.' . $phpEx);
to /bin/phpbbcli.php

This is a different issue from https://tracker.phpbb.com/browse/PHPBB3-14372, as it affects core installation / update, an not an extension.