Linux proper, ie the kernel, allows things like modules and has a powerful userspace. Most purposes are met by taking vanilla Linux and dropping some modules and userspace applications on it. You don't have to do a hard fork of the code.

But if you're talking about distros (or some vauge distro-like prepackaged blob of Linux + userspace), it does happen. There are a lot of specialized Linux distros out there, and they aren't all on long-term support with security backporting.