What is the status of a bravo airspace when the associated TRACON is evacuated?
In cases where a given unit of airspace (for example, a Class D circle around an airport with a tower) is only in effect part time, there is always a note to this effect in the FAA's "Airspace Designations and Reporting Points" document.  The current (August 2019) edition of this document is FAA Order JO 7400.11D.  The Class B airspace for KSLC is listed on pages B-25-26 and there is no notation that it is only in effect part-time.  In fact this is true of all Class B airspace.  Some Class C airspace is only in effect part time-- an example is Lincoln Airport (KLNK) in Nebraska.  However in such a case the airspace opens and closes according to a set schedule that is published in the Airport Facility Directory or in NOTAMs.

The Class B airspace at KSLC definitely does not stop existing when an unexpected circumstance closes the associated air traffic control facility.  In the absence of authoritative information to the contrary, you need to assume that you should not enter the airspace without a clearance.  Checking for NOTAMS would be one way to check whether the FAA has temporarily authorized some unusual procedure for the special circumstance you describe.  

In a NOTAM search via this link selecting an "archive" search (click on "location" tab for this option) and using "KSLC" as the search term, we see various NOTAMs for March 18 (ILS approaches unavailable, etc) but nothing indicating that any part of the Class B airspace was temporarily revoked.  Some of the NOTAMS key to messages that may be seen on this
Air Traffic Control System Command Center website.  It appears that none of the NOTAMs or messages pertain to alternative ways to get clearance to enter the Class B airspace.