I don't see the need for a private confession online or offline, but there's no way I'd do it online. You never know what digital trails you're leaving these days.
In a private setting with the pastor your level of trust is that the pastor won't repeat what you said, and isn't recording it. The trust is in the pastor. In an online setting you also have to trust that the connection is secure, that no other devices are listening in, that nothing is being logged etc. It's all but inconceivable that none of the servers between your computer and the pastor's computer are keeping logs.