The graveyard is full of indispensable people. I felt indispensable at my last job - company is still alive years later, and I wish I had left sooner. It's often hard to have perspective if you are The Person, or rely on The Person, but if the projects are important (Linux) they will evolve some kind of continuity and, if they aren't, they will die and other projects will replace them.

If you care a lot about a specific project and want to prepare then it's similar to preparing for any other disaster.

* Documentation and written processes for everything. People like to feel irreplaceable, but it's far more valuable to make yourself unnecessary. In high stress situations, people make bad decisions, so having well-oiled play books is important. E.g. see [0].

* Practice and simulations: at large companies people are sometimes put on surprised forced vacation and are not allowed to communicate with their team.

* Foster a culture of encouraging people to step out of their box - if Important People's decisions can be questioned, then the rest of the team will better understand them and will be able to cope better with making them if necessary

[0] https://www.atlasobscura.com/articles/pointing-and-calling-j...