This is what I mean when I say Vixens musings are like badly written pulp spy novels.
They aren't grounded in reality at all, just an ignorant layman's idea of how intelligence is performed fed by a diet of preposterous movies, tv shows and books written by people whose objective wasn't to illustrate the real workings of an intelligence agency but to be entertaining.
James Bond movies would be dull as anything if they were accurate to the actual workings of human based intelligence, even during the cold war. Sure, there is a limited amount of spy gadgets and sneaking around, but for the most part it's just embedding someone into a group or agency and taking notes of literally everything they do so it can be filtered and analysed by other people who might not even use it.
I read a story once of someone whose uncle was an MI5 operative. They were embedded into the Liverpool docks during the height of the Cold War in order to monitor the dock workers for Communist activity and any anti-British government sentiment. They weren't able to see or visit any of their family for over a decade, had to live on a dock worker's salary, not socialise with anyone outside of those they could reasonably be expected to meet as a docker, feeding constant reports to their handler detailing everything said during meetings with their targets and the union members. Eventually the Government withdrew the program and "pulled him out" only they didn't really, they just told him one day "don't need you anymore" and left him. Apparently the guy went nuts, started collecting newspaper clippings to do with the dockyards and doing the "connect disconnected events with red string" thing that conspiracists are wont to do because he couldn't or wouldn't believe he had wasted so much of his time.
Every classified document I've ever seen was not magicked away like it was never there. Usually the bare bones of the report are available and they just crudely edit out the "classified" portions of it. Or they have a second report with the bare bones written and just publish that.
Sure, the MI5 archives are full of documents that haven't seen the light of day outside MI5 but they will have at least some kind of link to something available outside, even if it's just people commenting that they can't talk about this thing that's happening or a cover story. Intelligence agencies aren't magic. They aren't able to whisk people away with invisible helicopters in the middle of the night. They still have to operate within the bounds of the law and oversight.