The single biggest skill that helps developers grow into leaders, is good communication.
Business leaders and managers grooming the next group of leaders need to focus on helping developers improve these skills.
These skills are learnt and don’t come naturally to most folks.
As a tech lead/manager, I introduced the concept of a meetup sprint.
Every quarter, or after a developer has worked on a major feature/project, I assign them a non-coding task. These can be any of the following.