Why is it not OK to kill off characters nowadays when I two off beloved characters since the beginning of storytelling?

It's quite OK to kill anyone you like if it's your book, and I don't understand the rest of the sentence.

I don't know what you mean by "not OK". If that's what your story calls for, then do it. There are no laws against it.

So far.

It's still okay to kill characters. I do it in about half of all the things I write. Last one I killed was a twelve-year-old.

The reader/listener often has expectations that any character who is portrayed as good is going to survive whatever happens. We all know that isn't how real life plays out, so killing characters who don't deserve it jolts the reader.