Is there a Accordion Panel in Vaadin 10 or is it yet to come or is it not going to be there in Vaadin 10 ?
No, there is not that kind of Layout component yet (neither TabSheet)
There is tabs navigation component though, so you can use that as building block for similar type UX.
Yeah, you can build a rudimentary accordion using vertical tabs. Place the content of the tabs inside the tabs component, between the <vaadin-tab> elements (yes, you can add arbitrary HTML inside <vaadin-tabs>), and toggle the visiblity of those depending on the selected tab.
There was an accordion in 8. Is it going to come back and appear in 10 sometime soon?