It is safe to pour boiling water down the drain.
Your pipes even when plastic are made to take the heat of boiling water and you won't melt your plastic drain pipes when pouring boiling water down the drain.
I've poured boiling water down my drain and kitchen sink several times when cooking and when just cleaning the drain.
It's recommended to pour a pot of boiling hot water down your drains every few months or so to help clean any potential clogs.
You can clean clogs in drains with baking soda and vinegar and then follow it by some boiling water to wash it down the drain.
The drain pipes will not melt or be harmed from the boiling water.
But just be careful to not splash boiling water on yourself and pour the boiling water down the drain slowly.