In a youtube video How to Fill Gaps In a Wide-Plank Wood Floor - This Old House https://youtu.be/7focJeab0pM Tommy cleans the gap and uses rope, non-synthetic, to fill gaps. I wonder if this could present a fire hazard like leaving stain rags on a job site. He does suggest staining the rope and letting it dry before pushing it into the gap. Does anyone have experience with this method?

That was hilarious! The old wood filler used when it was sanded and finished was falling out so I guess the rope trick could work.

Thanks for that!

Ropes wet with stain will burn if you light them. Spontaneous combustion caused by a pile of stain rags can not happen with rope. The heat build-up takes place by a chemical reaction as the oil starts to harden. The rope will not get hot enough to even start to smoke because it will give off the heat as it is created

I've worked on floors that had rope in them and filler over the rope . If you do that , try and find a smooth rope that doesn't look or feel fuzzy . The really hairy feeling ropes will usually be more difficult to bury so that you can't see he fibers poking through your filler .
Then over the rope , I might recommend timber mate brand filler . It's a water based filler that holds together very well and is less prone to shrinking and cracking .