You said it tangles gear, which is true, but that's just the start. Those old nets keep doing their job forever, which is called ghost fishing. They trap and kill fish, turtles, even whales long after they're lost. They also break down into microplastics that mess up the whole water. It's a full ecosystem problem, way bigger than just a hazard for divers.
Lost nets killing whales is a horror story few discuss. These things keep fishing long after they're gone, catching all sorts of marine life. The issue goes far beyond just a diving hazard.
We had some luck on a local reef clean-up. A few of us would go down with heavy duty mesh bags and shears. Cutting the nets into smaller, manageable pieces was key before stuffing them in the bag to haul up. Took forever, but it made a visible difference.