Is Minecraft for Raspberry Pi discontinued?

No, but it does not seem to be under active development.

As of 2014, it has been included in Raspbian and can be installed via:

sudo apt-get install minecraft-pi

It is no longer being updated.

I found this:

It ded :( The original team has stopped supporting it and starting with 0.9 MCPE became a lot more ambitious technically, which also means that it became a lot harder to strip down to run on a Raspberry Pi. Actually, we finished removing all Pi related code to reduce complexity in 2015. Maybe someday we'll revisit it but I wouldn't hold my breath :P On the other hand, Win10 runs on the Pi so you can run the Win10 version there! ... I guess it's something

from Tommaso Checci here.

The original comment on reddit can be found here.