The Atlas N-Scale Shay has been offered 3 or 4 times, and will probably be run again in the coming months or years.
As others have noted, they turn up frequently for sale on eBay and elsewhere. So if you want one now, previously-owned is the way to go. If you can wait for a while, Atlas will probably offer them again. They run well.
With the much wider variety of models in the market today as opposed to 10-15 years ago, nearly everything in N-Scale is a limited run. When sold out, there is a usually a wait before they are run again. But for those of us who started modeling in N in the 1990s or earlier, today's situation is much better. The presence of eBay and other re-sale outlets means that most models are never truly unavailable (though sometimes they are expensive). And there are very few models that won't be offered again eventually.