No one is indispensable, and Teams should avoid creating heroes. If this is a tendency, make sure to act early and to rearrange processes to prevent further worsening the situation. Do not treat the team member as the one at fault.
In most cases, this is a management error, although some people will always try to make themselves indispensable by hoarding knowledge. These persons are not making themselves any favors as they create a lock-in effect at their current role, making it impossible to promote them.
The idea itself is probably as old as time. However, the best modern reference in a DevOps context is found in The Phoenix Project.