hi hi
It turns out that it is actually quite difficult to hide a failed rocket launch. They tend to be remarkably visible events. Considering that even in the very early days of their space program, their failures were actually documented and now widely known, including the first human casualties in space. Like Vladimir Mikhaylovich Komarov, who died in Soyuz 1 on re-entry, April 24th 1967. They may not have published everything at the time, but they did keep internal documents, and a lot of those were released after the Soviet Union collapsed.
When it comes to politics, 2+2 can equal 6, but when you're trying to go to space, 2+2 has to equal 4. It's not like they could just pretend their failures didn't happen and succeed through the power of ignorance. Ignorance may produce hot air, but it doesn't produce delta v.
Even if we just look at the post cold war world, there is also still no way to reasonably hide a rocket failure, since the people who are trying to put a spacecraft into orbit generally want to know where it went.
The information is hardly classified.
You can look for yourself if you'd like.