Regards pre-Snowden, the situation was far more than an "open secret", there were multiple documented projects and methods employed. Among them:
- 2013: Hemisphere, an AT&T call records database dating to the 1980s accessible by the US DEA. (This was publicised near-simultaneously with Snowden's own 2013 disclosures.) https://www.eff.org/cases/hemisphere
- 2006: Mark Klein and the Room 641A revelations concerning AT&T's beam splitters: https://en.wikipedia.org/wiki/Room_641A
- 2005, William Binney and J. Kirk Wiebe, NSA whistleblower, warrantless surveillance. https://en.wikipedia.org/wiki/William_Binney_(intelligence_o...
- 2004, Russell Tice, NSA surveillance including of congressional members. https://slate.com/news-and-politics/2006/01/why-nsa-whistle-...
There were very strong suspicions around the TIA (total information awareness) and USA PATRIOT ACT (2003, 2001). I recall much chatter about this at the time, and the related FISA court, though little by way of specific details of technological measures and methods involved.
Carnivore, a WinNT workstation-based tool, disclosed ~2000. https://en.wikipedia.org/wiki/Carnivore_(software)
There are telephnic data retention programmes, including MAINWAY (revealed in 2006), containing an estimated over 1.9 trillion call-detail records, and
https://en.wikipedia.org/wiki/MAINWAY
Project ECHELON, with disclosures of varying aspects from 1972 -- 2000. https://en.wikipedia.org/wiki/ECHELON
There are earlier periods, notably addressed by the FBI's own COINTELPRO archives (https://vault.fbi.gov/cointel-pro), though for my purposes I'm considering "modern" projects to be circa 1990 onwards. (COINTELPRO and the Church Committee hearings resulted in substantial changes, at least publically, to US domestic surveillance).
And I've compiled a long list of pre-1990 references of concerns regarding significant technologists who'd warned of the risks of information technology as a tool of surveillance and control, largely as no such list seemed extant: https://toot.cat/@dredmorbius/105074933053020193
I'm well aware that documentation of clandestine and national security issues is difficult to come by, see the TK case for one reason why that is.
But that's also why specific documentation is so valuable and why I'd requested that.