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In PCI Express slot there is indeed a standby power (+3.3 V) pin and WAKE# signal pin, but it works differently than the power button.

https://en.wikipedia.org/wiki/PCI_Express#Pinout

The main difference is that WAKE# signal only restores power to the system but it does not change the system's power state, after using it the card needs to send an interrupt to the motherboard to properly wake it up. Power button does both things.

There is a pin that provides access to the 3.3v standby bus, and a wake pin that can be used to wake the CPU.

there is a WAKE# pin on the PCIe slot so any card should be able to wake the system from sleep (S3).

however, an ATX PSU supplies only 5V (5Vsb) when the system is off (S5) which is not available on the PCIe connector which only has 12V and 3.3V. so when the system is off (S5), the PCIe cards are unpowered and thus cannot turn the system on.

but if you mean off in the Microsoft "Modern Standby" (S0i) then yes, the PCIe cards are powered and the WAKE# pin can "turn on" the system.