I don't know about China, but in Russia private conversations do not trigger immediate response and they do not control every possible means of communication. They simply do not have capacity to investigate every violation - too many people talk negatively about the government and ongoing events, so use reactive approach. People may get in trouble while being searched on the border crossing or after being reported by someone, but it is hardly different from border searches in USA. Things may change with their new messenger and disruption of WhatsApp and Telegram there (Russia just started blocking SMS verification codes making registration there difficult).
There's literally a white list of permitted sites now, supposedly only to be used when there's a 'drone threat'. Guess what, there are places in Russia where there's a constant 'drone threat' for at least half a year and vk.com is basically all they can use to communicate. Why would they start arresting people for private VK messages now, while their 'max' messenger is still struggling? It could wait until all other messengers are less than 10% market share, that way it won't impede adoption until it's the only option available.
They don’t need to monitor every conversation. Just enough that every conversation is a little risky. It’s the ability to read it all, if they want, that matters.
As far as I know, their biggest problem isn’t reading chats (if device seized and unlocked, not a problem at all regardless of service and encryption level), but listening encrypted calls. This one really bothers them and WhatsApp appears to be threat number one. I don’t know anything about the scale of CSAM distribution there, but I think they don’t need ChatControl-like technology for dealing with it. ChatControl was worse than Russian surveillance state, maybe on par with Chinese tech.