Its never been against regulation but various councils stipulated it wasnt to be done to avoid confusion with the switchwire if done in black/blue...... it was word of mouth that spread through the industry but with no real backing so much so that a clark of works on one site i did told us to remove the Neutrals, i had to download the relevent info before he'd believe me and presented it to him with a job spec asking for switch mount buzzers to be installed when key switch was operated for e/m lighting, after he made us feel incompetent in front of customer it was nice to have him eat humble pie..... didnt get any hassle after that from him.