The Official Secrets Act is probably bad law, it's one of the many places where the USA has superior laws that are much better at keeping bad government in check. The various attempts to expand censorship in the UK are also very bad, in my view. The D-Notice system is difficult to really get worked up about because, as this exposé makes clear, it's:
a. Voluntary.
b. Sometimes ignored, even by major outlets like the Guardian.
c. Largely made up of journalists themselves.
d. So shadowy that they publish photos of themselves and upload meeting notes to government websites.
e. Apparently very tightly concerned with stuff like publishing the names of spies, soldiers, troop movements, etc. The classical sorts of information that has a very direct and obvious reason for being kept out of the press.
The article tries to build a case that it's being used in over-broad ways like with Assange, but honestly I don't buy it. The media doesn't need some shadowy committee to stop it covering the Assange story properly. At some point it became received wisdom in media circles to hate him and journalists need no incentive to bury stories if they find them inconvenient. The Guardian in particular has a long running and well documented vendetta against Assange. You'd see the same approach if D-Notices existed or not.
I think that puts it very mildly. There was a gigantic public scandal as far back as 1967 about D-notices.
https://en.wikipedia.org/wiki/Super-injunctions_in_English_l...
I could offer anecdotes, but my interactions with my social circles more and more indicate 'well, duh' to stories like this and not post-1981 book reading horror of realization that humans can be quite horrible to one another. And, well, since it is clear now that some information is suppressed and some is heavily promoted -- the distrust of general populace appears to be quite high.
And this is yet another reason why I constantly argue against the temptations of censorship. That said, this is the path we are on.
So to answer your question. No, I don't think it is sinister at all. It is just the current state of affairs. Some subjects are more risky than others.
It doesn't help that it struck me as drawn-out and disjointed.
Worse - I never got the sense that the public would much care if the facts that the DMSA is keeping secret could be freely published in Britain. Nor (post-Snowden, and having read about some of America's secret gag orders) that the UK is up to anything all that bad (by the modern-day western world's very low standards).