I definitely came across it multiple times in the Harry Potter series. First of all, yes, it is correct and used in the older books. Second of all, no, it isn’t really usual nowadays, though back then it was more usual. Third of all, it can be understandable, but I guess it depends on the readers age or education level, or if they ever came across it. Personally, when the parenthesis is used, I always thought the person was muttering the thing inside the text, for example, “Your parents died from a serial murderer (which I think you probably already knew), but strictly speaking, you cannot tell this to anyone” (just a random dialog thing I made). Finally, you can use commas instead of it, for example: “Your parents died from a serial murderer, which I think you probably already knew, but strictly speaking, you cannot tell this to anyone. Honestly, IMHO, I don’t recommend you using them, merely because many people do not know what it is, and will become very veryconfused, as I did. this is probably not going to be read by you as it is late, but I hope others find it and benefit from it Hope I helped you, reader!
EDIT: I also found it in the older Charles Dickenson books (which I didn’t read, just because I was surfing the net and saw that)