It is not safe. Here’s why:
You could ruin the pipes: If you have PVC pipes, as most people do, boiling water can melt the seals and cause harmful damage. If you have a porcelain toilet bowl, boiling water can cause this to crack.
Clog runs deeper: You may think boiling water could break up a clog. Wrong. It could actually just move the clog deeper.
Injury: Lastly, you risk burning yourself since boiling water can splash back onto you.