Not just closing all apps, but also disconnecting pendrives safely, for exemple.
Does holding down the physical power button make Plasma try to close all apps before power cuts off?
No, and I would strongly advise not doing it.
Not creating a wishlist, or holding down the button?
holding the power button for a few seconds generally results in a hard reset.
it would be equivalent to turning off the PSU or unplugging the machine.
the OS does not get a chance to respond so whatever you have in ram is lost.
that said it may be entirely possible to set up your PC so that holding the power button sends a shutdown signal which will safely close apps and sync USB drives
but i would not recommend it because then you have no way to issue a hard reset when need to.
Does normal shutdowns disconnect pendrives safely?
Mounted devices are umounted on a shutdown.
Normally a short press of the power button will start a shutdown.
A long 6 or more seconds of the power button forces the power off with no shutdown.
Systemd allows you to configure what the power button does for the short press.
The long press is implemented in the motherboard and I don’t think can be changed.
Ever heard of REISUB? If not, look it up. But, this is basically what you are asking to be added as a feature when pressing and holding the power button. It’s a great idea, but mostly only as an idea, as I imagine there needs to be BIOS/kernel settings involved to make this work correctly.
Instead, you should try to use REISUB if your computer is frozen, especially since 9.5 times out of 10, it will shut down your device safely.