Hello,
I have the question, if chunks behind the World Border are still being generated if someone is near the border.
If not I would like to know if it is possible to add that.

Thanks.

It is not possible to cancel chunk generation behind the border. Set your border size accordingly, take other actions such as installing a void generator plugin (to generate air there) or setting the player's view distance low enough to not generate chunks.

If you are using a web map you can also utilize the visibility limits feature to prevent the map from displaying chunks past the border. You might need to re-render the map afterward.