Secrecy of correspondence doesn't necessarily only apply to physical letters as far as Constitutions go. In Finnish constitution it is defined as "The secrecy of correspondence, telephony and other confidential communications is inviolable" meaning it also applies to any internet message.

Unfortunately large majority of parties in Finnish Parliament do not really care about that provision and have passed multiple laws which create exceptions to it. They do it via the proper protocol (which is essentially the same as modifying the Constitution itself) so it's technically legal.

Secrecy of correspondence still has exceptions. That's what is always lost in these discussions -- every right of every person is not absolute. Just because you have a right to personal property, doesn't mean you don't have to pay taxes or store nuclear material in your basement. That's the hard part.

But end to end encryption with forward secrecy at no cost to user makes your right to private communication absolute. It's a new thing and the balancers can't balance it against other rights of other people, so this happens.