Make sure to update your package manager and other packages.
I've updated both, and even restarted, but the version remains the same. My system doesn't know any newer versions:
protobuf-compiler is already the newest version (3.6.1.3-2+rpi1)
root$ sudo apt install plocate
Did you perhaps mean
sudo apt install locate? There's no package by the name "plocate". Also, I don't get anything in return when I run the locate command. Maybe it only works on OpenSUSE? Absolutely nothing:
root@raspibolt:/home/bitcoin# locate libprotobuf
root@raspibolt:/home/bitcoin#
(I've installed the locate package)