What is the electrical connection of heatsink plate of PB case bridge rectifier packages?
The "Enhanced Iso-Cink(TM)" legend on the datasheet, though not fully explained, may be explained in other Vishay documentation. It seems to mean "Isolated Case/Heatsink" and the datasheet says "Dielectric tested ... to withstand 1500V".

One way to use it then would be to mount the heatsink to chassis ground, if the equipment has a safety ground connection. Then any breakdown will isolate the equipment by tripping breakers or fuses, and the heatsink will not carry dangerous voltage in case of failure.

The alternative would be an isolated heatsink; in that case, it must be inaccessible, because in case of breakdown it could reach dangerous voltages.