Yeah, it's true. When something really bad happened (in other words, someone very important had died), all TV programs were cancelled, and you had to watch something extremely boring instead.It wasn't always ballet.

Sometimes, it was just a still picture with autumn leaves, and sad music in background. So, ballet wasn't the worst option.

Why? I believe, there were two reasons. First one, Soviet people had to feel sad, when something bad had happened. It'd be strange if newly widowed woman was happy, right? It's the same logic here. Soviet people can't have any fun if something is bad. They have to feel sadness.

(And Soviet TV wasn't extremely fun in the first place. Especially, daytime programming).

Second, TV executives needed time to decide how to tell this sad message to Soviet people. So, there weren't any real news yet, but you already had to be sad, or you had to be ready to be sad, so there is your ballet.

I believe it began with Brezhnev's death (1982). After that his successor died (Andropov, 1984). And after Andropov's death, his successor died (Chernenko, 1985). So, you know, it became a trend.

Except that we can't know for sure it was a ballet they showed on days of these leaders' death. There are a lot of memoirs about these days, and mostly you can find mentions of TV ballet, but sometimes authors says that it was something else, something boring and sad, but not the ballet. I, personally, believe it was a ballet, and it was a Swan Lake by Tchaikovsky, but I can't think it could be easily proven, because all we really have from these times are old papers with TV lists (and these TV lists doesn't contain any ballets, they contain items that were cancelled).

So, there is a relatively small chance, that it's a Mandela Effect, because we have another example of ballet on Soviet TV. It happened in 1991, during SCSE coup d'etat attempt (ГКЧП). During these days, all the Soviet people could see on the main TV channel was Swan Lake, interrupted by short news coverage. Three days in a row.

And the fourth time was a charm. Swan Lake became a legend.

(I have to say, that this time the situation was slightly different. First, at the beginning no one died. Second, Swan Lake was on TV lists before the events, it had to be shown on 19th of August. But instead of one-time show it was shown for three days, because TV executives didn't know what to do.)

If you want to know why ballet was considered appropriate for tragic events, or why it was considered boring enough, I don't have an answer. Personally, I love this music (but don't care about ballet).

Some details about 1991 ballet (Russian)