Swallowing mucus is not gonna harm your health or kill you.
However it is bad to swallow your mucus anyway because your mucus contains bacteria and other germs that your body is trying to get rid of.
When your body is sending mucus up through your throat it's wanting to get rid of that mucus that contains those germs and bacteria.
But when you swallow that mucus back down your throat those germs and bacteria get sent back into your stomach and body.
So if you're sick with a cold or flu and are coughing up mucus but swallow it you're just hindering the time it takes for your body to heal.
It's important to spit that mucus out so your bodies immune system can work to fight and get rid of those germs and bacteria so you'll get feeling better sooner.