The term "keeper" is used for two distinct things here, and you seem to be confusing them.
The keeper referred to in xapp197 and UG156 is used inside the FPGA fabric on the unconnected inputs of LUTs, etc. It's a "half latch" used to stop those unconnected inputs flapping about in the breeze. There are also some radiation tolerance issues (since the keeper may flip in a way that can't (easily) be detected, possibly changing the functionality of the design).
The keeper referred to in UG974 is a configuration of the IO buffer weak pullup and pulldowns. It only applies to IO pins. It's not really related to the other type of keeper at all.
That said, I don't know if the first type of keeper is used inside Ultrascale. I have only seen documentation that says they are used up to Virtex5.