Security issues can occur in both the device-agnostic Android OS code and in device-specific drivers. Some people are going to continue to use phones after the vendor drops support. Although those people won't get security updates to their device-specific drivers anymore no matter what they do, by using LineageOS instead of their stock ROM, they will get security updates to the device-agnostic Android OS code.
No, it isn't. It's good for reducing e-waste, your security is on the line.
Having the newly shaped and colored Android UI doesn't do anything to fix security issues..