How do does KDE Plasma keyboard shortcuts work? Maybe it's just something supported for KDE applications. Despite the k at the beginning, ksnip doesn't belong to KDE, it just uses Qt like KDE.

It's using the Desktop Entry Specification. In particular the "Additional applications actions": [Desktop Action .....].
This should also be valid and work with any other desktop environment.

For example, I have modified the desktop file to create these actions:

$ cat ~/.local/share/applications/org.ksnip.ksnip.desktop
[Desktop Entry]
Type=Application
Exec=ksnip
Icon=ksnip
Terminal=false
StartupNotify=false
Name=ksnip
GenericName=ksnip Screenshot Tool
Comment=Cross-platform screenshot tool that provides many annotation features for your screenshots.
Categories=Utility;
Actions=Area;FullScreen;Window;AreaDelay;

[Desktop Action Area]
Exec=ksnip -r
Icon=ksnip
Name=Capture a rectangular area

[Desktop Action FullScreen]
Exec=ksnip -m
Icon=ksnip
Name=Capture a fullscreen

[Desktop Action Window]
Exec=ksnip -a
Icon=ksnip
Name=Capture the focused window

[Desktop Action AreaDelay]
Exec=ksnip -r -d 3
Icon=ksnip
Name=Capture a rectangular area with delay 3s.

Now if I add the application to the KDE shortcuts I see this:

That's nice, it looks like it's not directly related to plasma. Can you create a PR with the default actions that ksnip support (excluding for example the one with hardcoded delay)? Nice would be also a wiki entry on how to create your own actions in case someone want to do that.

Done PR: #451

About wiki entry is easy:

Copy .desktop file from system to user: cp /usr/share/applications/org.ksnip.ksnip.desktop ~/.local/share/applications/
Edit user file (~/.local/share/applications/org.ksnip.ksnip.desktop) and add unique name for the custom action in the Actions= field of the .desktop file and add the new action to the end of the user .desktop file:

...
Actions=Area;LastArea;FullScreen;Window;<NewActionName>;
...
[Desktop Action <NewActionName>]
Exec=ksnip <option to execute>
Icon=ksnip
Name=<Descriptive name of the action>

Wiki entry added https://github.com/ksnip/ksnip/wiki/Desktop-Integration

Thanks again for implementing this :)