The button "Convert To Work Order" on the WebUI Opportunity form is not visible anymore with Field Service 8.1x.

With Field Service 7.5x. it was still visible. However you can find this button now on the Opportunity form of the new Unified Interface.

When I try to add the button back to the WebUI Opportunity form with Ribbon Workbench (copying and pasting from a 7.5 solution) it doesn't appear.