You can document institutional knowledge and try and make sure nobody is indispensable but ultimately you will still take a huge blow. You should focus most of

You can document institutional knowledge and try and make sure nobody is indispensable but ultimately you will still take a huge blow. You should focus most of your attention on improving retention. I left a full 200 page notebook full of product ideas/customer notes/requirements/designs when I left my last job. Hopefully it helps the team but it is only in note form and completely unordered. More likely they won't look at it and the knowledge will just be lost.