This sounds like a Tim Cook aphorism (right before he hands the iCloud keys to the CCP) — not anything with any real legal basis.
> No one shall be subjected to arbitrary interference with his privacy [...]
which has later been affirmed to include digital privacy.
> I don’t think any government endorses that position.
Many governments are in flagrant violation of even their own privacy laws, but that does not make those laws any less real.
The UN's notion of human rights were an "axiom" founded from learned experience and the horrors that were committed in the years preceding their formation. Discarding them is to discard the wisdom we gained from the loss of tens of millions of people. And while you claim that society has a vested interest in violating a terrorist's privacy, you can only come to that conclusion if you engage in short-term thinking that terminates at exactly the step you violate the terrorist's rights and do not consider the consequences of anything beyond that; if you do consider the consequences it becomes clear that society collectively has a bigger vested interest in protecting the existence of human rights.
“Arbitrary” meaning you better have good reasons! Which implies there are or can be good reasons for which your privacy can be violated.
You’re misreading that to mean your privacy is absolute by UN law.
But the "arbitrary" there is too account for the situation where the democratic application of the law wants to inspect the communications of suspected terrorists, and where a judge agrees there is sufficient evidence to grant a warrant.
Unfortunately, that law does nothing against situations like the USA/Russia regime where a ruler dispenses with the rule of law (and democratic legal processes too).
You can't practically have that sort of liberalism, where society just shrugs and chooses not to read terrorists communications, those who wish to use violence make it unworkable.
That is arbitrary interference with all our privacy.