How do you mitigate the impact of unexpected talent departures? Are there any tech solutions or other strategies you use to retain institutional knowledge?

Creating an environment where trust is the foundation leads to effective information sharing and thorough documentation of all activities. Additionally, as David mentioned, it's crucial to always plan ahead and explore new domains that you haven't considered before. For instance, there's a growing trend where professionals from finance, who possess core skills, are transitioning into cybersecurity. They often fit well in Governance, Risk, and Compliance (GRC) roles due to their expertise and willingness to adapt.

When someone decides to leave, there's often little you can do to change their mind. However, you can plan ahead by continuously hiring, rather than waiting for an open position to arise. This proactive approach ensures that you always have a pipeline of potential candidates ready to step in when needed.

You must invest and plan in advance for documentation, information sharing, training, coverage plans, etc. before attrition occurs.

Simplying taking time periodically to assess the team and see if you have any areas that are only served by one person is a great start.

Ensuring you document and bring awareness to this is next.
Show your team that there are areas with a single-point-of-failure, help raise awareness of the true risk.

The decision can be to mitigate via hiring backup or spreading actions/work across other people, OR it can be to accept the risk and do nothing.

Beyond that, try to have some sort of knowledge sharing and management solution.

Pick a tool, have team members document critical knowledge and if possible document standard process and procedures.

I have found that if we can show a risk (single person handling critical operations) and they have taken time to document processes, it can make the ROI discussion on hiring backup easier as the new hire can jump in quickly and ramp up via documentation versus having to shadow for days/weeks.

Transparency is your friend in this.

Finally, take time to see how your team is doing.  Don't be caught off guard by someone leaving because they have been stressed, unhappy or over/under worked for a long time.  Take time to know those around you.