If I have a prediction in a sealed envelope, I sometimes give the spectator the choice at the end:
to open or keep the envelope closed and therefore never to know whether the prediction has come true or not, so that it will remain a mystery forever.
This method is not mine, but I once read it somewhere
Is there anyone who knows who first published or used this (Phil Goldstein?) ?

Not sure who came up with the idea, but curious what the impact would be on the audience if the individual chooses to keep it a mystery? What would it do to the strength of the effect?

I really, really like this idea. My first contact with this (and I am certain NOT its origin) was Chris Ramsay's "Red Pill". There it is very effective.
You have demonstrated to the spectator that an impossible object probably has been created by magic. The draw of human curiosity versus perceived value of a sealed box is intriguing.

I believe it was Jon Allen who first put this idea into print in his book, Experience: The Magic of Jon Allen. He was inspired by J.J. Abrams' TED talk

https://www.youtube.com/watch?v=vpjVgF5JDq8

The "Mystery Box" discussion was in an issue of Wired Magazine where J. J. Abrams discussed how he keeps the item he bought unopened as a reminder of the power or mystery.
https://gizmodo.com/the-jj-abrams-issue-of-wired-5220590
Though you might also like to look at the movie Repo Man

Doing a card trick for someone as if you are just repeating something you did for them a while ago and giving the envelope back to them as if they asked you to keep it... well ... ( This is where a blank card could also come in handy )

That's a very interesting idea I never came across! I wonder if we could combine it with an effect like Turner's Chinese cookie. Either you read it now, or you can keep it for later...

I wonder what the date is on the first performance of the Penn and Teller segment where they give the audience the choice to close their eyes and be mystified, or keep them open and learn the secret.