I've heard that during the space race era, many of the Soviet cosmonauts were lost in space. Or at least, such were the claims of some. So I wonder, is there any truth in these claims, or was that merely cold war U.S. anti-communist propaganda against the Soviet Union?
Has it really happened, for example, that cosmonauts would become untethered during extravehicular activity and were lost in orbit or to outer space?
As the Wikipedia article you refer to says, there is no conclusive evidence any cosmonauts were lost, and most of the 'lost cosmonaut' stories have been disproven.
The evidence cited to support Lost Cosmonaut theories is generally not regarded as conclusive, and several cases have been confirmed as hoaxes. In the 1980s, American journalist James Oberg researched space-related disasters in the Soviet Union, but found no evidence of these Lost Cosmonauts. Since the collapse of the Soviet Union in the early 1990s, much previously restricted information is now available, including on Valentin Bondarenko, a would-be cosmonaut whose death during training on Earth was covered up by the Soviet government. Even with the availability of published Soviet archival material and memoirs of Russian space pioneers, no hard evidence has emerged to support the Lost Cosmonaut stories.
In the book 'Two Sides of the Moon', co-written by David Scott (Apollo 15) & Alexei Leonov (first to walk in space), Leonov discusses the Bondarenko accident. but does not mention anything about lost cosmonauts.
When the Apollo 1 accident is mentioned, Leonov expressed his surprise that the accident happened given that the Bondarenko accident occurred a number of years previous to the Apollo 1 launch pad accident. Leonov wrote that he thought the Americans would have known better because those involved with the Soviet space program thought the Americans had spies within the Soviet program and they would have told the Americans about Bondarenko and why a 100% oxygen atmosphere was a bad idea.