There are many alternatives to Onboard on-screen keyboard for Linux if you are looking for a replacement. The best Linux alternative is Maliit Virtual On-screen Keyboard, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to Onboard on-screen keyboard and three of them are available for Linux so hopefully you can find a suitable replacement. Other interesting Linux alternatives to Onboard on-screen keyboard are Florence Virtual Keyboard (Free, Open Source) and Kvkbd (Free, Open Source).
Maliit provides a flexible and cross-platform input method framework for mobile and embedded text input, including a virtual keyboard. It has a plugin-based client-server architecture. Clients can communicate with server via input context plugins.