This was initially discovered by WABetaInfo earlier this year and recently came to light again after security researcher Athul Jayaram shared it on his website. According to Jayaram, this issue came about because WhatsApp did not ask Google to ignore indexing certain links which contained phone numbers.
Jayaram claims that Google provides tools to web administrators that allows them to let search engines, such as Google, know that they should ignore indexing certain links, and as a result, phone numbers of users were accidentally listed. Thankfully, WhatsApp claims to have already fixed the issue, although it seems that Jayaram will not be getting a bounty for his efforts.
According to a statement made to TechCrunch, a WhatsApp spokesperson said, “While we appreciate this researcher’s report and value the time that he took to share it with us, it did not qualify for a bounty since it merely contained a search engine index of URLs that WhatsApp users chose to make public. All WhatsApp users, including businesses, can block unwanted messages with the tap of a button.”