WASHINGTON—The IRS said Thursday it plans to hire 10,000 new workers to help reduce a massive backlog that the government says will make this tax season the most challenging in history. The agency released a plan to work down the tens of millions of filings that includes speeding up the traditionally slow hiring process, relying more on automated processes and bringing on more contract workers to help with mailroom and paper processing. Getting it done will be the big challenge, tax experts say. The agency faces a backlog of around 20 million pieces of correspondence, which is more than 15 times as large as in a normal filing season, according to the agency. And the IRS workforce is the same size it was in 1970, though the U.S. population has grown exponentially and the U.S. tax code has become increasingly complicated. Additionally, the need to administer pandemic-related programs has imposed …