As for "retroactive prosecution": Is there such a thing? I think not. Investigation and detection of "illegal" activity is timeless insofar as prohibition of a behavior is codified. Even if that behavior is NOT codified, the default recourse to injury is litigation. Such is the dynamic of a social contract purportedly dedicated to jurisprudence and lawful arbitration.

One of the articles in the Universal Declaration on Human Rights covers that

Article 11.

(2) No one shall be held guilty of any penal offence on account of any act or omission which did not constitute a penal offence, under national or international law, at the time when it was committed. Nor shall a heavier penalty be imposed than the one that was applicable at the time the penal offence was committed.

But you point out that this "telco immunity" affects civil litigation against the telecommunication companies, not possible criminal liability.

And on this note, I think constitutional protections are intended to protect citizens from the State, not from each other. It was the government that was in violation of the 4th amendment, not the telcos. The telcos may have been in violation of data protection laws if the US had any, or of their own privacy agreements entered into with their customers.

When the capital development of a country becomes a by-product of the activities of a casino, the job is likely to be ill-done. — John M. Keynes