On my copy of Windows 8.1, it's still there in the same place it always was, Control Panel, (search box:) Keyboard. There are sliders for repeat delay and repeat rate, in the same old dialog box as always.

Strangely, I can't seem to find out how to reach that dialog box without using the control panel's "search" box to find it. If I try to navigate the hardware menu by hand, I can't see an option for the keyboard. Note that I have that same problem on both Win7 and Win8.1, so this isn't a Win8-specific thing.

Note: If you're having trouble finding the old-school control panel at all in Windows 8.1, you can make a shortcut to CONTROL.EXE.