What gives you the impression pardons are limited to those that are "delivered" and/or "accepted". How would that even be legally demonstrated to a court.
The Supreme Court apparently ruled that way in a case a long time ago and that precedence has never been challenged (or come up again for that matter)
See https://www.youtube.com/watch?v=c9IRZ9FzWEA
But realistically if they tried to revoke a pardon today it would certainly go to the Supreme Court again and they would almost certainly rule the other way.
Looking up Grants revocation of Johnson's pardons - it's hard to find details, but it does seem like a Federal judge took issue that they were not delivered prior to the new president revoking them.
Given that there's no such limitation in the Constitution, I wonder if it would hold up to SCOTUS. ...and maybe that's why they announce them publicly these days.
This is literally all it says...
> he shall have Power to grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment.
I think a more interesting question is whether there's a specificity requirement. AFAIU, that hasn't been answered, either.