On scaffolds, on building sites, kick boards are quite rightly mandatory. They prevent tools or materials being knocked off to fall on people working below.
When bridges cross rivers this is not relevant
When bridges cross other paths it is good practice but is not always observed.
If the kick board is in direct contact with the deck it will trap debris. Soil will build up and plants establish. The combined effect will keep the wood permanently wet and the life of the timber will be reduced,
On equestrian bridges The British Horse Society recommends high kick boards - partly to reduce the horses awareness of whatever is going on under the bridge!
The arrangement of rails to maximise protection to small children is also a consideration