How do you build up a pool of software engineers who could step into a leadership role and fill a gap after a departure?

To build a pool of software engineers ready to step into leadership roles after someone leaves, train them early with real tasks, like leading small projects. Give them mentors to guide them and chances to learn skills like decision-making. Keep a list of who’s ready to step up. This way, you’re never stuck when a leader departs!

You have to keep an eye out for people that are already helping others in the team. The ones they look to for guidance, and then let them try it. Find opportunities for them to lead a feature release, or a small project. Help them work with other teams and build their networks. Do coffee roulette with them and see which ones want to do it. 
You may also want to set up a mentorship program and see who is a good mentor and who seeks out a good mentor. That will help you provide an environment for them to practice coaching other. 

For that, it is really important for you to understand which members of the team want and have the leadership skills to fill that role. In my experience, many Engineers rose from being in an IC role to a Management role, just because it was required to fill that role, but without asking them if they wanted to be part of a Management track rather than a Technical track. Being in a track where you can't find your passion will result in attrition, frustration, and resentment. Instead, being in a track where you have found your passion will bring satisfaction, motivation, and drive.

In meetings, I make a point to listen carefully for potential talent. I often engage in casual conversations to gauge interest or aptitude, without explicitly revealing my intent. Once I identify someone with potential, I begin assigning them tasks and offering guidance to support their development. This approach helps me evaluate their capabilities while fostering growth in a low-pressure environment.