This is probably true. Most Beijing taxis are missing rear seat belts, usually shoved somewhere. Given Beijing traffic, it is quite scary, especially when the taxi driver looks like they are at the end of a 20 hour shift.
When I was working in Beijing, the seatbelts were usually there, but tucked underneath the seat cover (and so inaccessible).
I'm from the UK, where using a seatbelt is pretty much universal, and when getting into the cab I'd automatically pluck at where the seatbelt should be.
Once when I was taking a ride with our agent there, a local, and he saw me do this, he told me off quite sharply for it...
You should use didi zhuanche in Beijing, it's like Uber and the vehicles are nicer (Buick or high end VW usually in Beijing) without seat covers...so the seat belts are always usable. They also give you free bottled water. A bit more pricey than the taxi, but maybe living is worth it.
