I have child seat in my back seat secured to designated attachment points (not the seatbelt) and its heavy enough to warn me EVERY time. It's secured, not dangerous, and is a persistent false positive. Badly designed systems can also do harm by training people to ignore them or even actively circumvent them.

This. Even outside crash situations, a brusque movement could send things flying, distracting the driver enough to cause Bad Things.

Use the boot, people; at worst, put stuff under the seat, never above.