What This Means
The 400-point tier is limited, but it proves the point: Alaska is not reserved for the mid and upper tiers in 2608A. If you can live with Interior - GTY and a very small sailing set, you can get into Alaska at the lowest tier in the release.
That said, 600 and 800 points are the more useful Alaska tiers. At 600 points, Alaska appears repeatedly, including Voyager of the Seas sailings from Seattle. At 800 points, Alaska becomes one of the strongest regions in the tier, with Serenade of the Seas and Voyager of the Seas both showing up in the top options.
So the answer is simple: Alaska unlocks at 400 points, but it becomes practical at 600 points and better at 800 points. If you are aiming for the best balance of access and choice, 600 points is the first tier to watch.