Comment 14 for bug 1637878

They removed VLAN support in KDE's network manager in KDE Plasma version 5.9.0

Read here:
https://www.kde.org/announcements/plasma-5.8.5-5.9.0-changelog.php

A developer thought that no one will be using VLAN configurations on a desktop machine, because he doesn't use it. Thus he removed that feature on 2017-01-06:

"Don't allow to create bond, bridge, vlan and team connections
These connection have never been properly tested and supported and they
are not used on desktop anyway. Since we already filter these out, there
is no reason to allow to create them."
https://cgit.kde.org/plasma-nm.git/commit/?id=aae4b7886bcf865d976e98910e24b13436521f8f

I suggest everyone under this bug report writes him an email and tell him, that he is wrong about his assumptions. Thus he has a reason to reimplement that feature again.
His email can be read in the commit message of that change.