I think this is good advice from Allison Rossett re: capturing knowledge of retiring employee. I’m afraid there are too many databases created waiting for a miracle to happen that magically populates them.
Start with a strategic area of need and invest there, by collecting to advance that important goal. Focus not on generic retiree participation, but on addressing an ongoing priority. Ask specific questions. Add human touch… people who interview, recognitions for contributions, podcast interviews on key topics, exec sponsorship and participation… What you probably cannot do is to put up a database and expect them to populate.