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Google to start permanently deleting users’ location history
(www.theguardian.com)
This is a most excellent place for technology news and articles.
The data has costs associated with it: they'll want to back it up, they need to migrate it when they change formats, they need to maintain the hardware it resides on.
And, as the article mentions, there are liabilities around law enforcement requests, costs due to data breaches, and regulatory requirements.
Three months is plenty for them to target ads.