AFAIK the idea was to label different types of plastic and let people figure out how to recycle them later. But it's now decades later and some types of plastic still can't be efficiently recycled.

Most plastic can't be recycled, yep. If it would melt under low heat (plastic bags, food containers, other packaging) it can't be recycled.

HDPE plastic -- the heavy duty stuff? The only truly recyclable plastic.

I throw all plastic that can't be recycled in the trash.

Lack of prevention.