There are old, now mostly useless blueprints of old satellites, (or rockets, or pieces of software, or engineer's notes, e.t.c) but they are still closed to public. Why? Can it harm the Space Agency which manufactured them somehow?
- Rockets (even old designs) are capable of delivering a nuclear weapon anywhere on Earth. Governments don't want this technology to fall into the wrong hands.
- Publishing a design takes a lot of effort. The complete design for a rocket easily exceeds a million drawings and hundreds of thousands of pages of supporting documents. For an old rocket, all of that only exists on paper or microfilm, so it'd cost millions to scan it all and make the documents accessible.
To address the comments:
Lots of information is available to the public. You can buy entire books on various space projects (I've seen books on the Shuttle, the Apollo project, the Curiosity rover for example).
You won't find much if you search for "satellite blueprint" in your search engine. The word 'blueprint' has a very specific meaning: this is a copy of an engineering drawing using a contact print process on light-sensitive sheets, i.e. a copy of one drawing showing one individual part.
What you're looking for is a diagram or a cross-section. The Flight Global archive is a good source for those, e.g. I found a diagram of the Apollo-Soyuz Test project from 1975. Or enter the words 'Soyuz diagram' into your search engine and do an image search.