I've been flying for a long time. It's only in the last decade that I've noticed the specific language of "keep your seatbelt fastened even if the light turns off". It's always been a "soft" suggestion, but it's becoming stronger, and repeated. The ANA flight crew in particular were quite adamant about it.
This may just be me becoming more aware of it, but, if so, I'm also becoming more aware of more passengers actually heeding the advice. I remember, not that long ago, most people removing their seatbelts as soon as the light went out. Not so common now.