What advice would you give to a senior software engineer/developer who is on the fence about transitioning into leadership?

Depends on the definition of "leadership". Assuming the engineer has already been a team lead and was good at it, or they made their team more successful then we are looking at technical leadership, ie: architect etc or management leadership, ie: Engineering Manager. If you enjoy working with a set of teams and then doing whatever it takes to remove roadblocks so that they'll succeed then you should probably give it a go, if your management style is to dictate the plan and then your teams have to roll with your decision then you should stay away from any management positions.

Leadership allows you to learn about the latest solutions, what works well and how this outdates existing solutions. If you are an established developer, then the knowledge you acquire in a position of leadership will only cement your credentials as you would have a strong understanding of how things work. Everyday new AI solutions are born but do they address the real challenges of software delivery would be something you can lend your voice to. Leadership does not mean you lose your appetite to learn but it means you are now in an all you can eat buffet, and your passion drives you try everything.