This is a distribution to you of unrecoverable basis in Roth 401(k) contributions because the value of your original Roth 401(k) had dropped below what you had contributed.
If the Tax Cuts and Jobs Act of 2017 had not suspended miscellaneous deductions subject to the 2% of AGI limit for 2018 through 2025, the amount in box 5 could have been deductible on Schedule A. With that suspension, there is no deduction for the unrecoverable basis. (Even if these deductions had not been suspended, you would still only benefit from the deduction if your itemized deductions were more than the standard deduction. See Losses on page 23 of IRS Pub 575:
https://www.irs.gov/pub/irs-pdf/p575.pdf
