Guard Band Reduction in 5G compared to 4G
(i) Why was 10% of bandwidth was chosen as guard band for 4G?


That 10% was probably seen as a realistic value at the time. Realistic in the sense that it could be achieved with the available technology at the time.


  (ii)How has it been reduced in 5G?


Note that the 5G standard doesn't say anything about how to implement the system, instead it describes how the system should behave. So the 5G standard doesn't tell us how to reduce the guard band it just tells us what it needs to be.

Again the value is likely to be technology related. Technology has moved on so now we can have more processing power available to shape the signals such that a smaller guard band can be used.

The shape (over frequency) of a 4G or 5G baseband signal is determined by how much (digital) filtering is used. Obviously more complex filtering could result in a signal that is "cleaner" so that the distance between channels can be smaller and smaller guard bands can be used.