Some northern cities has heavy metal or impurity problem due to geography or industry.That won’t reduce heavy metal/chemical contamination, I’ve read that in some cities it’s absolutely fine, but also that Shanghai water is possibly dangerous, but I’m also just having a hard time finding testing results.
Shanghai has few very very old blocks with outdated pipelines.
If u are traveler, those northern city is no hot spot, also those outdated Shanghai places.
Most likely you will go to most modern part of mega city, which has new pipelines.
And those hot travel places in southern provinces have the best water quality such as Zhangjiajie and Guilin.
The tradition of drinking boiled water is a heritage of war time, people just keep it but dont know why.