It was disabled in 2011 since KDE 4. But even years later, it seems there is still interest in having this plugin.

Commits of removal:

• git commit e443c58df03e9fb8f26b67e86852f708d097517a

Details

• Bug report: https://bugs.kde.org/show_bug.cgi?id=299248
• Blog about removal: https://kate-editor.org/2011/08/09/maintainer-needed-kate-external-tools-plugin/
• Rewrite from someone in 2016: https://bitbucket.org/ZaWertun/kate-plugin-externaltools