While never "safe", lead pipes were relatively
safer before the epa started pushing chlorine alternatives.
High concentration chlorine (and flouride, where used) were reactive to contaminants such that they were often chemically/ electrically attracted to the lead pipes (clay soil and other environmental factors played a role), and the pipes leeched contaminants from the water. Lower concentrations of chlorine and any concentration of most chlorine-alternatives not only allow water to leech from the lead pipes, but it also can reverse the previous process, such that decades of leeched chemicals and contaminants deposited in/on the lead (and other) pipes are released back into the water, at a much higher rate and concentration.
Washington DC went through this issue [0] in what was a huge public embarrassment, at least within the civil engineering community. It left the EPA with a black eye in what was supposed to be a landmark and example-setting project.
Now that it's known to happen, better precautions can be taken, but it's absolutely still an issue, especially in places like Detroit, where the surviving lead pipes were largely the service lines on the user side of the water meter. (Remediation and upgrades to water distribution systems often stops at the meter, including design considerations.)
[0]https://en.wikipedia.org/wiki/Lead_contamination_in_Washingt...