I think you have a misunderstanding of the system.
Sealed indictments are not new. All indictments are sealed after the Grand Jury returns a True Bill. Indictments must be unsealed before an arrest warrant is issued.
While the indictments haven’t yet been unsealed, Trump also hasn’t been arrested, nor has an arrest warrant been issued.
Finally, an arraignment (the first Court appearance where you are formally notified of the charges) will never happen with a sealed indictment.
The 6A protects you from being tried on secret charges.
However, when being arrested the police *are not* required to notify you for the reason for your arrest.
So, nothing novel is happening in this instance related to the sealed indictment. The thing that’s unusual here is the case is so high profile we know about it after the Grand Jury returned an indictment but before it was unsealed.