It is a very niche path to dive into if needed. Typically reserved for those who work for the companies that compile them.
However, you could find yourself in a role where developing custom kernels is a part of the job description. A couple years ago we piloted a new requirement that created a custom kernel to meet a customers need with their very specific hardware device.
That kernel was stock but added device and other hw lookups that made this specific hardware 'work' out of the box during boot and then performed updates remotely. So, incredibly niche. These admins were later repurposed to OS support roles. Which is where they were brought from mostly, save for a couple new hires.
Focus on OS support. RHCE was most of their certs but some had a role where a cyber security degree was a suitable sub.