Umm, at least in my opinion, there are quite a fed red/golden sex-links out there that aren't very easy to sex at hatch. . . That doesn't make them not sex-linked. A bird really easily sexed at
hatch is actually considered an auto-sexing bird.
Otherwise, to me, the only sex-links truly easy to sex at hatch are "black" sex links.
Chris is right on the dot. I think everyone knows that Sex-Linking is just an F1 thing, plus there's plenty breeds out there with enough colors to make just as successfully easy to sex offspring as do you find with hatchery sex links, and beyond that, the purpose of it is within the breeder's goals.
True sex-links, genetically, is simply through the parentage. Now by marketing terms, yes, if you cannot sex them at hatch, they would not be considered sex-links when sold. When I got some Shamos, they were sex-linked offspring. I considered it so because it was genetically so, even if you couldn't tell upon hatching due to the solid yellow wheaten coloration.
Love the idea though! There are a lot of breeds with enough colors to make a "purebred" sexlink, but sadly not many people care to obtain the breeds for such. Most people with colors of breeds beyond what hatcheries offer are in it for, well, non-production and profit, so, making sex-links is kind of out of the question. But, here's a little taste of my nonintential sex linked Polish -
Tolbunt Polish x Silver Laced Polish hen = males hatch out white w/creamy pinto spots on body. Females hatch out mostly brown and black with white spots on body. As they mature, the females will look either like Gold Laced or Tolbunt. The males will look either like creamy golden (as in, light yellow) laced birds with possible mottling.