It was used before 1918 - it was first standardized in the Civil Script (1710). But it wasn't considered a separate letter until 1918 - so e.g. the standard alphabetic sorting ignored the distinction. For this reason, it wasn't always used consistently, although it was still much more consistent than Ё. And even today, "иод" is still considered valid spelling; indeed, it's the preferred one in scientific context.

This still shows up in some contexts - e.g. Й, like Ё, isn't used in bullet lists; try it in Word - it'll go from И straight to К.