Recently, Nintendo/The Pokemon Company "asked" Pixelmon to shut down. Which they did.
Is it still possible to download and play Pixelmon on pc?
The official Pixelmon mod has been shut down. However, a quick google search revealed Pixelmon Reforged, found at https://www.reforged.gg, which can be downloaded through the Technic launcher:
DISCLAIMER: I do not provide any guarantees towards the lawfulness, quality, stability and/or security of the Pixelmon Reforged project. This is just what I found during a quick Google search. It might work, it might not, and it might also be taken down by Nintendo soon.