Kernels are supported for a decently long time. As far as I know, it takes pretty much no effort to maintain older kernels, just a little extra hard drive space in the repos to keep them and their drivers there.
Upgrading can result in security fixes and performance differences.
I'm not sure if you get a message, but I've never heard of anybody ever having any issues of running a no-longer-supported kernel.
I realize I didn't quite answer your questions, but I guess my point being is, it's nothing to worry about.