The petition is open to all EU resident. The goal is to replace all Windows in all public institution in Europe with a sovereign GNU/Linux.
If the petition is successful it would be a huge step forward for GNU/Linux adoption.
The insecure parts of Linux is mostly on the DE side opposed to the core OS part that servers use. We absolutely will see more vulnerabilities in the future as Linux grows.
Many developments over the last few years have been for improving those aspects, e.g. Wayland is far more secure than X11 could ever be. There will be more vulnerabilities found, but it won’t be as bad as one might fear.