Kde Panel Launcher Files Location?

On Debian 9 'Stretch' with KDE Plasma 5.8.6, this is now in:

~/.config/plasma-org.kde.plasma.desktop-appletsrc

The remaining info is as provided by @TheBlackCat

It is in the text file ~/.kde4/share/config/plasma-desktop-appletsrc.

You want to find this section: [Containments][n][Applets][m][Configuration][Launchers], where n and m are some numbers you need to find out on your system. There should only be one Launchers section for each task manager you have.

If it isn't there, you should look through [Containments][n][Applets][m] for one containing the application you are looking for.