I have lost the ability to rebind the screenshot key in the options. The printscreen key still works, but I used to use the F5 key instead. This is a new problem as I was able to take screenshots using F5 just last month in May. If the option has moved to another section of settings I can't find it, and I looked thoroughly.
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I'm not sure when, or why, the keybind was removed. You may be able to set your own screenshot key by editing the cvars.xml file directly. Look for the screenshot bind in the file.