What the setting does is enable memory mapped I/O for a 64-bit PCIe device to 4GB or greater address space so that there is no shortage of address space or collision with address space reserved for other uses, such as ROM, RAM, Other I/O, Sound, etc. If memory mapped I/O isn’t an option then address space must be kept in the below 4gb address space, which can run out fairly quickly.
Above 4gb decoding should be an option on many Intel and AMD based boards. Not all boards need or include the option based on how expandable they are. Generally those designed for more than one video card will include it. Think motherboards with 3-x16 slots or 6 or more total slots. This is because each card put into the slots needs a certain amount of I/O space and lots of slots translates to the need for more address space.
If you have a motherboard with limited expansion capability, for instance a single x16 and a x1, then it’s possible the board doesn’t need the option.
To try to answer your questions.
You should update your BIOS to the latest version anyway.
Your MSI board looks like it will support one video card normally. I don’t know your board personally, but people online say they are using GTX 1060s and even higher end cards. With 2 riser cards, it looks like it should support two 1060s and should run them at full potential.
With 2 cards it is likely you will not need above 4gb decoding. My Asus board runs 3 or 4 cards before I have to enable that.
It is likely that your board will not support that 8x expansion card you have shown. I would go with 2 riser cards. Those expansion cards with 4 or 8 usb on them can work, but take lots of effort and the exact right motherboard, settings, etc.
Which processor do you have on the board now? A10?