With the API shutting down, I believe there is no longer an automated way to delete all content.
Actually, the API hasn't shut down. It's just you get a bill if you go over 100 api calls per minute, but existing scripts like github shreddit can be easily modified to include a builtin delay to prevent that from happening. Alternatively you can pay shreddit.com $15 to do this for you and not worry about it (they use their own API key i figure though I don't know the specifics, but I imagine they have a setup that prevents them from going over the limit as well).