I defer to folks with expertise in derogs and in legal issues, but if the CA really has indeed been shut down, and its accounts were not sold to someone else, then it seems like the following should work. Simply ask Equifax to verify the collection. Its standard proces of reaching out to the entity reporting the data (the CA) will end with no response (since the CA no longer exists) and by default the collection will be removed from your report.
That assumes that your primary goal is to see it removed. If you have an even milder goal, which is simply to avoid harassment or attempted collection, then you can do nothing, choosing not to stir the pot by a dispute.
As I say, other people may know better what to make of this. Curious what they say.
If they are no longer in business, then the collection authority on your reported collection must have been terminated.
CRA policy, as set forth in their credit reporting manual, the "Credit Reporting Resource Guide," instructs a debt collector to delete any collection if their collection authority has been terminated and the debt remains unpaid.
Send the CRA any proof you have that they no longer have collection authority, and request deletion per their reporing requirements.