When preparing new versions of Plasma 5.8.x the main efforts of the developers is aimed at improving stability, and increasing the functionality of the given secondary importance.
27 December was the fifth release of the KDE Plasma 5.8.5 LTS, which is made of about 50 different fixes and improvements translations.
If you are using Linux Mint 18 KDE environment with the Plasma 5.8 and KDE neon, the developers recommend to update to version Plasma 5.8.5.
Schedule for subsequent releases of KDE Plasma 5.8:
KDE Plasma 5.8.6 - 21 February;
KDE Plasma 5.8.7 - 23 May;
KDE Plasma 5.8.8 - 17 Oct;
And the latest release of KDE Plasma 5.8.9 will be released on 10 April 2018.
Installation KDE Plasma 5.8.5 LTS into Linux Mint:
You need to add the repository
sudo add-apt-repository ppa:kubuntu-ppa/backports-landing sudo apt update
then you must fully upgrade the system via terminal or Update manager
sudo apt full-upgrade