Making horns non-mandatory at crossings wouldn’t prevent the engineer from using it when appropriate.

Here's over an hour of recordings of level crossings (railroad crossings) in the UK.

I've obviously only watched a couple of minutes, but there are no train horns. There's a mixture of urban and rural locations.

https://youtu.be/9FoeIxfYW20?t=122&feature=shared

For pedestrian crossings at least, I did a long distance walk in England where there was signage at a couple crossings to the effect of look, listen, and be aware.

On other walks, there was a phone you were supposed to use (I did) to call an operations center to get a go ahead to cross.