It is possible to use "is" in that sentence, but it wouldn't be common. It would have to be apparent that there was a third time reference between the time of writing and now for it to make sense. That isn't built into the sentence as it would be for example if you wrote "What he had written became true and is still true today". Without that third time point, you would write simply "What he wrote is true".

You can write "what he had written was true" or "what he had written is true" but the meanings are different. It has to do with narrative distance from the event. I will explain below! 1. "What he had written was true." > > > The whole story takes place in the past. He said something, and it was true at that time. You are telling the story now, but, both what he said and the thing that was true were / occurred in the past. 2. "What he had written is true." > > > The story takes place at two different time periods. He wrote something in the past, but it refers to something that is true today. You are telling the story today, talking about something that is true today, referring to something he said in the past. Hope this helps!^^

You can also put emphasis on the word "was" to specify that what he had written was true in the past (at the time he wrote it) but is no longer true.

I would say "What he had written IS true" is theoretically possible, but no one is likely to say it in reality. For that, "What he wrote is true" is shorter, clearer, and more natural - you don't need the perfect tense. If you google "What he wrote is true" (with quotes), you get about 91,000 hits. If you do "What he had written is true", you get a single hit, and even that one is irrelevant, saying altogether different thing with a comma after "written". Just remember that the tenses are not maths. People tend to use the simplest tense that can express the intent clearly.