I've never generated an API key, and Linode showed it as blank. I generated a new one anyway, but I can't imagine how they'd have a default key and somehow not show it in the UI.
Edit: Groan, here's their clarification. It's starting to look like they don't know what the heck they're talking about:
"Thanks for getting back to us. To be extra cautious it would not hurt to regenerate your Linode API key. You can do that in your user profile. Please let us know if you have any other concerns we can address."
The Lish password is set to a random string by default, however we would still recommend resetting this password even if you had not set one manually previously.
I had expected that if the password was not set, then password auth was disabled. I've told them that's what I want and have asked when it will be implemented.Now it sounds like basically everyone should have reset their API key. Bleh.