There is no requirement that anything pertaining to SOL be mentioned in a DV.
SOL is your basis for defense if sued, and is your obligation to prove in court. Ultimate ruling on expiration of SOL requires a finding by a judge.
It is possible, in limited circumstances, that expiration of an SOL is apparent from the facts of a case, such as a very old debt where the state SOL is much less, and has no reset provisions under your state law based on any other factors, such as offers to pay or actual payments made on the debt.
The only reason I would mention possible expiration of SOL in a DV is if you feel the facts of your case clearly establish its expiration.
In such cases, there is case law holding that if a debt collector is aware that the SOL has clearly expired, then any threat or initiation of legal action can become its own violation of the FDCPA. If you are sure, it can establish basis for notification that might reap benefits in any subsequent legal action or complaint.
If unsure, I would leave it out.
The expiration of SOL is usually clouded when a debt collector may be the plaintiff. The SOL applicable in any trial is that of the state of jurisdiciton of the trial.
The FDCPA gives a debt collector the option to bring legal action in either the state where the contract establishing the debt was signed, or in the jurisdiction of current residency of the consumer. Thus, one would not know the applicable SOL for any legal action until the jurisdiction of the legal action was established.
Probalby best to just leave out any reference to SOL in a DV, which is sent for the purpose of requring them to cease further collectin on the debt until they provide verification.
