In the situation where we have: It used to be thought that...
We use the past tense with the verb that follows since it's something in the past. But what if the following is meant to be a fact?
I came into a question that says: It used to be thought that the Earth is flat. It used to be thought that the Earth was flat.
Aside from the piece of information in the sentence, it represents a fact(Earth is flat) which makes the tense simple present. I believe that the correct one is is, am I right or not?
Thanks in advance.