No, they did not extend the deadline for 2018 returns.  The following year, they extended the deadline to July 15 because of Covid.  

The IRS began accepting electronically filed individual tax returns for the 2018 tax year on January 28, 2019. This marked the official start of the 2019 tax filing season.

How much worse could the next filing season be, considering all the cuts made to IRS already this year?  I don't think the shutdown will make much difference.  Fewer people means fewer auditors and fewer CP-2000s.  Less revenue, sure, but deficits and dollar devaluation are always good for the economy and there's always an election coming up. 

I found it interesting that IRS is reminding its furloughed employees:

 "As a reminder, the Employee Assistance Program is available for all IRS employees and their immediate family members at any time, day or night, by calling 800-977-xxxx (TDD: 800-697-xxxx). This no-cost counseling service could help address stress and other issues you and your family may face. Visit GuidanceResources.com to register for an account to help you improve your work-life balance. Get access to counseling, coaching, health and wellness, financial, legal and other work-life resources. Use organization web ID: IRS112. You can also download the app at GuidanceResources® Now."