If an employee claims more than 10 allowances on their Form W-4, does the employer have to report this to the IRS?

No, this requirement has been eliminated. However, Forms W-4, Employee's Withholding Allowance Certificate, are still subject to review. Employers may be directed (in a written notice or in future published guidance) to send certain Forms W-4 to the IRS. The IRS will also continue to review employee withholding compliance. The IRS may send you a letter (commonly called a lock-in-letter) specifying the withholding rate and allowances to use to calculate the amount of tax to withhold from wages paid to a specific employee.