Should people be prepared for interviews with a portfolio of career work? If that portfolio is lost in the mists of time, what should you do?

Having a portfolio at the ready is always helpful (and can be a nice thing to reflect back on for you). For product marketing, this can consist of things like blogs or web pages you've written, webinars or conference sessions that you've presented, etc. I recommend always keeping a running Google Doc of links to the ones you're especially proud of as they happen. (Reminder, make sure these are externally published and that you're not saving any internal materials from past jobs) If you don't have these materials, you can always offer to do a writing sample or otherwise for the specific interview.

However, regardless of whether you have the materials available or not, being able to speak to the projects clearly is what matters most. Be prepared to speak to what the project was, why it mattered, what made it a success (or not), and the role you played in making it happen and what specific assets you owned/created. While the outputs of a launch are great, what's often more important in an interview is showing how you got there and how you navigated any challenges.