Some companies like to be cheap and not hire sufficient number of engineers. Companies are fine understanding redundancy for their hardware and their services etc... They totally failed to grasp it for their employees. That's all fine and good if you're putting in a resignation and giving two weeks or a month notice but often times things are not planned. Family emergencies happen where you have extended times people can be out, grave injuries or death happens or people can just be gone the next day.
So however you plan for those eventualities is the same way you plan for and manage the knowledge transfer on resignation.
Any one-off knowledge transfer will most likely fail. People can't learn actionable knowledge with doing the actions. - That's why we need continuous sharing and doing.
People only look for the artifacts of a knowledge transfer when they need it. When resolving unfamiliar issues, people's first question is: "Has this happened before? How did we resolve it the last time?" It's a simple question but most often its very hard to answer.
The knowledge needed to answer questions like the one above, is mostly manually compiled together and often from memory.
Finally, actionable knowledge is hosted in the same tools as product specs, meeting notes, press releases etc. As a result, creating, finding and following such knowledge is quite hard.
Disclosure: I'm building Savvy(getsavvy.so) to fix a lot of these issues for developers.
You can't do a brain dump of an employee and have it be effective. The key to doing knowledge transfers is to make them unnecessary by making it a built-in ongoing process that people don't even notice.