The nRF51 products do not appear in the 'Products' list on the Home Page:
Are they deprecated, or "no longer recommended" ?
If you go via the 'Products' link right down at the bottom of the Home page, the nRF51822 is there:
But not nRF51422 nor nRF51824
The nRF5 SDK link from the nRF51 product pages does not take you to the appropriate SDK version, and does not mention that the latest version is not suitable!