Not illegal, but since other sanctions went into effect, it seemed obvious that even rich people’s assets are becoming fair game in this new hot/cold war we’re having.
It’s possible to do business in Russia, maybe even legal. Is it a smart thing to do? It’s the equivalent of the people travelling to Russia or NK for tourism, and finding out they will be made into hostages. The wealthy just thought that they are above the war, that the war only takes poor people’s lives, not rich people’s money.