The Stasi did mail tracking on a massive scale in the GDR before the wall came down. Funny/Sad how things seem to go in circles.
Which funny enough is also happening in the US, but has nothing to do with mail tracking and everything to do with approval of no-knock police raids and general apathy to ever doing something about obvious corruption and brutality by people like Arpaio.
Who in turn has absolutely no access to any of said surveillance infrastructure to start with...
Also the great east berlin telephone line distribution node: 50 wires coming in, 100 going out ... only 50 of those were for households.
All of this can be seen in the Berlin Communications Museum.