I am going to throw caution to the wind here and make a statement that I am sure many others will say is wrong, but here goes…
Copyright is NOT a verb!
There I said it. It’s an adjective and noun.
We put something under copyright. We protect something with copyright. (noun)
My book is copyright protected. (Adjective)
The problem with copyright as a verb is that it doesn’t really work in the past tense. It becomes an adjective.
My book is copyrighted. My book was copyrighted.
Let’s look at the meaning of the ‘verb’. It means to undertake an activity (usually registration) to protect something from plagiarism.
I copyrighted my book means I protected my book from plagiarism. The meaning is clear = In the past, I protected my book. Therefore: in the present, I have not protected my book. In other words, the meaning in the past tense of the verb is not the meaning (my book is protected under copyright) that the speaker/writer wants to convey.
Best to say…
“My book is/was protected under copyright” and not try to ‘verbise’ a noun.
Note: Verbise (to make a verb from a noun) does not exist.