I wrote it a long time ago, made it depend on phpbb\db\migration\data\v320\dev for no specific reason (it's probably what the migration I used as a base depended on) but there is an abnormally high number of users who have apparently not run it and I don't know why. It could also be that some users somehow skip some migrations for reasons unknown.

The file is there, it's only 15 lines: https://github.com/phpbb/phpbb/blob/master/phpBB/phpbb/db/migration/data/v320/allowed_schemes_links.php#L16-L31

I should add, no need to actually review the file or spend too much time on it, take a quick look and close that ticket if it seems ok.