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Hot socketting means that chip have very good latchup protection. It means that every chip pin can survive large (100mA and more) latchup currents.
I didn't find any hot socket declaration for Xilinx devices but they declare latchup currents 300mA for FPGA devices and 200mA for EPLD devices.
Another problem with hot socketting is what will happen with power-on reset, register states and configuration SRAM after plugging. I think that you can fast plug device but after that you need to manual reset device or switch on/off power. So in this case hot socketting is not so significant advantage.
“Cyclone® II devices offer hot socketing (also known as hot plug-in, hot
insertion, or hot swap) and power sequencing support without the use of
any external devices. You can insert or remove a Cyclone II board in a
system during system operation without causing undesirable effects to
the board or to the running system bus.”
This article is found on the ALTERA'S Cyclone II Device Handbook.
I think it can answer your question.