It was documented in Vaadin 8 version
https://vaadin.com/docs/v8/charts/java-api/charts-advanced#charts.advanced.export.exporting
The API and functionality is still there.
It is important to note the part
"The functionality uses a HighCharts export service by default. To use your own, you need to set up one and then configure it in the exporting configuration as follows:"
This might come as surprise if not well documented.