In an effort to reduce identity theft, the IRS issued proposed regulations that would permit employers to use truncated taxpayer identification numbers (TTINs) on Forms W-2, Wage and Tax Statement, issued to employees (REG-105004-16).

Permissible TTINs are Social Security numbers (SSNs) with the first five digits of the nine-digit number replaced with asterisks or XXXs in the following formats: ***-**-1234 or XXX-XX-1234.  The effective date of the proposed regulations is Jan. 1, 2019.

Forms W-2 to be submitted to the Social Security Administration or the IRS are not applicable to this change.  This is to ensure that accurate information is received by the authorities.

But TTINs may be used on Forms W-2 being provided by employers to employees that report third-party sick pay because that is permitted by statute. Truncated numbers may also be used for reporting wages paid in the form of group term life insurance.

(source: https://www.federalregister.gov/documents/2017/09/20/2017-19910/use-of-truncated-taxpayer-identification-numbers-on-forms-w-2-wage-and-tax-statement-furnished-to).