APPSDK download page still seems to be broken. I'm checking with the concerned team.
As a workaround, you can download a temporary copy of APPSDK installer from here: AMD-APP-SDKInstaller-v18.104.22.168-GA-linux64.tar.bz2 [ Dropbox - APP SDK ].
As I've been informed, the concerned team has decided to drop the support for APP SDK and hence, any related package on developer central might not be accessible anymore.
With AMDGPU-Pro and ROCm drivers, you don't need a separate SDK because required libraries are shipped with the driver itself. And the headers to compile can be found with the package manager. For example on Debian based distro:
sudo apt-get install opencl-headers
Adding to dipak, I found this page very useful:
Basically you now need only the driver (I suggest you install it with secure boot off),
when installing use the flag if
With Vega10 or newer:
With GPUs older than Vega10:
Then you should be able to find your platform as AMD GPUs with clinfo.