In some cases we need to delete all private conversations from an old user but without deleting the user (so it doesn't break continuity in public conversations).
We use an SQL query:
SELECT DISTINCT Name FROM Channels INNER JOIN
(SELECT Id FROM Users WHERE Username LIKE 'Foo')
sub ON Name LIKE concat('%', sub.Id, '%');
Then "platform channel delete" to delete the channels.

But it would be much easier to have a CLI command that does this.