While the IRS doesn't state specifically, since you now have until July 15, to make a 2019 contribution to your IRA. I would think the same applies to corecting excess contributions to your 401k.
For more information, please see these articles:
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- Coronavirus Tax Relief [IRS]
No, the the change in the tax return filing deadline does not change the April 15, 2020 deadline for a 401(k) to make a corrective distribution of an excess elective deferral. The deadline is specified as April 15 or an earlier date specified in the plan agreement, not the due date of the individual's tax return.
Ultimately, it's up to the plan, not the IRS, to determine the deadline.