And interestly, WeChat claims they don't store user messages, but Chinese police apparently has it. Any app, website or forum on which users can post comments must verify ID-linked phone numbers first.
Does it really matter if it is PSB who stores it? :)
Special police in every major city have a few hundred people whose full time job is to go through online correspondence. That's not even a secret.
And I say this with the direct opposite intent of whataboutism - it's horrifying in China, and if we let it, it'll be just as horrifying here.
Compared to the human life cycle, quantum cryptography is at the "on the verge of speaking in coherent sentences, but still prone to smearing feces on the wall" age and stage. You can sense that they'll grow up to be quite smart, but there's a lot of learning to go before you can hope for a Junior Einstein.