This is what the delete button was meant for, and why pencils still have erasers. The length of this cyber statute of limitations can be set as arbitrarily as any other. In light of the free expression risks I perceive, I suggest the length should be short perhaps 6 months for an isolated message. If, to the contrary, there was an objective continuation of the challenged conduct, or a continuing pattern of wrongful acts, the cyber statute of limitations would be tolled as any other. This suggestion is feasible.
Computers internally record the date on which a document was created.
Once the limitations period has passed, documents should be legally consigned to the cyber wastebasket. My solution is imperfect. But so are humans. If perfect recall defines perfection, computers have achieved it. But their operators have not achieved it with them, and humans are unlikely to do so.