can't see zero-k games in Spring Lobby

SpringLobby doesn't support the Zero-K server unless that was implemented unbeknownst to me. I think you're looking at the Spring lobby server.

zero-k is not available on linux steam/ invalid platform error appears when I try to install it.

We can't release officially on Linux until it runs out of the box on SteamOS. That needs an engine update which hasn't happened yet. Until then you'll either have to ask Histidine on Discord to PM you a beta key or use the itch.io version. The only difference in the steam version is that you can play the campaign in coop mode.

Yes, the game is not officially available for Linux on Steam. You need to either use a Beta-Key which allows you to install the game on Steam, or use the non-Steam version from itch.io.

Is there any particular reason we need to ask for key manually? Can't this key just be displayed on web site in download section?...

Not every Linux user will know or want to know about command line stuff. We have a manual install page, but looks like it's not linked to from Downloads any more: https://zero-k.info/Wiki/DownloadLinux
I agree with you on that the only way to install Zero-K on any linux distribution is having a list of things to install in addition to Zero-K.exe, and then describing all the many ways how to install those things.
I'm sure that we would be happy if you wanted to contribute install instructions for Manjaro on that page.

For Debian-based distros we also needed to install these things:

mono-complete
xprintidle
p7zip-full
wget

Since some maps are 7zip files, wget was used for some downloads, xprintidle for debugging, and simply "mono" did not include all of the parts of Mono that are being used. Also, if you have no games installed before Zero-K, we also needed to install SDL2 ( libsdl-2.0 ). Are you sure these are not needed on manjaro? Do you have them installed?

Also, before the itch.io client, I had a script that would install ZK for Linux. You would run it after extracting it to a folder, which would then become the game folder. It does not depend on any debian stuff, should work on all Linux computers. However, it does not know how to install the dependencies except on Debian based distros.

installdir=$( dirname "${0}" )

# User install folders
lshare="$HOME/.local/share"

# Where to place .desktop file
apps="${lshare}/applications"
# Where to place icon
icons="${lshare}/icons"

# Setup dependencies ...
pkgmanager=$( which apt-get )
pkx=$( which pkexec )
if [ -n "${pkgmanager}" -a -n "${pkx}" ]
then
${pkx} ${pkgmanager} -y install mono-complete libsdl2-2.0-0 libopenal1 libcurl3 zenity libgdiplus
fi

# Binary name
bin="Zero-K.exe"

# Setup ZK...
cd ${installdir}
installdir=$PWD
wget -N https://zero-k.info/lobby/${bin} 2>&1 | tee /dev/stderr | sed -u "s/^ *[0-9]*K[ .]*\([0-9]*%\).*/\1/" | zenity --progress --text "Downloading Zero-K Lobby..." --title "Downloading Zero-K" --auto-close --auto-kill --no-cancel
chmod +x ${bin}

# Create .desktop file for launching Zero-K from the menu
echo "[Desktop Entry]
Type=Application
Version=1.0-5
Name=Zero-K
Keywords=game;strategy;
Icon=Zero-K
Path=${installdir}
Exec=mono ${installdir}/${bin}
Terminal=false
StartupNotify=false
Categories=Game;StrategyGame;
Comment=Free real time strategy (RTS) game
" > "${installdir}/Zero-K.desktop"
chmod +x "${installdir}/Zero-K.desktop"

# Verify local folders exist
if [ ! -d "$apps" ]
then
mkdir "$apps"
fi

if [ ! -d "$icons" ]
then
mkdir "$icons"
fi

mv "${installdir}/Zero-K.desktop" "${apps}/."

mv "${installdir}/Zero-K.png" "${icons}/."

# Delete itself
zenity --info --title "Done\!" --text "Zero-K is now installed\! You can find it in your applications menu."
rm "${0}"

I made an update, check it out here: https://zero-k.info/Wiki/DownloadLinux