Please consider the following:
- I knew the building (IS/WAS) thirty feet tall.
- They realized I (AM/WAS) his son.
My question: both those clauses contain general truths. So, do I need to use the present, or the past?
If the clause after knew or realized is a general truth, or more specifically, if it is still true, we can use either the past or the present.
This website, talking about backshifting, says
Backshifting occurs not only with indirect speech, but also with reported feelings and thoughts expressed frequently with verbs such as know, think, realize, and forget.
In certain situations, the sequence of tense rules are relaxed and backshifting is not required. Essentially, backshifting is not required if a statement about the present or future still holds. . . .
So the rules for verbs like realized and knew are the same as the reported speech: if the fact is still true, then you can use either the past or the present. In speech and writing, we usually use the past unless there is some reason to emphasize that the fact is still true.
If you want an actual example in print, in New York Magazine, in 1972, Mario Puzo says about Francis Ford Coppola:
That's when I knew he was really a director.
Coppola never stopped being a director. And Mario Puzo presumably spoke English well: he was born in New York, and is the author of the book The Godfather, and coauthor of The Godfather screenplay, which Coppola directed.