Parts of the US Government have been shutdown since December 21st, 2018. The IRS has had limited operating ability. Criminal cases are being conducted. Very recently, the IRS, Automated Collection Systems (ACS), has reopened and is handling levies, working out payment plans on back taxes due, etc. If you are receiving IRS letters, you or your accountant can sort out your issue with ACS as of mid January 2019. Revenue Officers, on the other hand, are not working UNTIL government fully reopens.
If you believe you meet the criteria for an Offer in Compromise (OIC) on your back taxes, you can still send in your OIC, although it will take some time to process.
If you are hoping to get your tax return in for 2018 in or your older returns, you can still file. Refunds? You should expect some delays.
