"It was not safe" is correct and usable in written English.
You can use this phrase in a variety of contexts. For example, you might use it in a sentence like, "Although the playground looked fun, it was not safe."
"It was not safe to stay inside this building".
He tried to stay there but was ordered out because it was not safe.
(Sellers, a licensed captain, had insisted that it was not safe for ocean travel).
If you were gay, it was not safe to be yourself.
After this statement, Firtash said he decided it was not safe to travel.
Considering his past at East Lake, Singh knew it was not safe to celebrate.
But he felt that it was not safe for him to return.
In Louisiana, fewer than half of the participants said they thought it was not safe to eat gulf seafood.
Her doctors told her mother, Cathy Morelli, that it was not safe for Melissa to go home.
"Before 1989 there was a feeling that it was not safe to say, 'I am a Jew,' " Rabbi Schudrich said.
It was not safe for the family to remain in Syria – they were all targets for the regime.
