Wiredtapping and postal interception, as well as metadata (pen-trace and postal covers) are possible, but scale poorly when individual lines must be listened to by individual agents, or individual letters carefully opened and resealed.

Digital permits surveillance at mass scale. It seems ultimately a fundamental property of the medium, less a bug than simply a feature.

There is also a fairly robust tradition of privacy in postal mail (in most countries), and after some false starts, eventually applied to telephony, at least in theory. The situation for email is far less evolved.

These days, if you do want secure communications, postal probably offers some real benefits. I'm somewhat surprised that postal remailing services (send an outer message to a central point who deposits the enclosed prepaid inner envelope(s) to final destination(s)) isn't a thing, or at least not one that has any appreciable awareness.

The capabilities of voice-to-text and handwriting / optical character recognition make the viability of intercepting virtually any spoken conversation, or any _observed_ written communication, quite high. The costs are much greater than with straight machine-readable character text (ASCII/UTF-8/Unicode), but pretty tractable.

My view is increasingly that privacy is an emergent phenomenon responding to ever-increasing surveillance and observation capabilities. The modern discussion began in the 1890s (Warren & Brandeis: https://www.cs.cornell.edu/~shmat/courses/cs5436/warren-bran...), as technologically-mediated intrusions were increasing greatly in capability. Though what the end-game is I do not know.