This article was written before the government reopened. The reopening is only temporary, for the next three weeks, so these problems may arise in the future.
The tax overhaul of 2017 will affect every tax return filed for 2018, and tax season starts Jan. 28. At this moment the IRS will have approx. 46,000 of its more than 80,000 employees at work. This years start date is consistent with last years filing season, so not much has changed in that capacity. The agency has said it will re-open call sites that had been closed during the shutdown. But even those wont be fully staffed and wait times could be extremely long.
This tax overhaul affected tax brackets, expanded the child tax credit and eliminated or limited several niche breaks.