After installing Preview 2 of Visual Studio 2019 i noticed that R# menu wasn't shown in the menu bar any more. The first thing i have done was to run R# setup and repair the installation. After starting Visual Studio again, the R# menu was still missing. But this time i loaded a solution and i could see that R# is still working. Taking a closer look to the menu bar i noticed a new menu named "Extensions". There i found R#*s menu.
It seens that MS had decided to bundle all extension stuff in a separate menu. Unfortunately they didn't mentioned this in their relase notes.
Hope this helps if someone other stumbles over this "feature".