It’s probably less about making the kernel smaller and more about security and reviewing code. The less code you have to maintain, the fewer vulnerabilities even if it’s old code.

I would doubt almost 20 year-old code is taking up a lot of space or presenting new vulnerabilities. And it’s obviously open source so if anyone needs it, they can always use an older kernel or maintain it. Sometimes, your oldest code is insane. I wish there was a budget for every company and government to pay retirees part time to go back over their oldest code that’s still in use. A lot of retired programmers would do it for fun and nostalgia. And to be horrified something they wrote 20 years ago hasn’t been updated or replaced.