There is very little that is difficult about doing this with the right software and hardware behind it.
When we have a weather crisis is my part of Florida, the county or city calls with a recorded message. And an Amber Alert rings on all cell phones when necessary. Sending an email to a few hundred thousand workers strikes me as an easy thing to do. Ask the company that sends out emails to see if I want to buy new sheets...
My wife’s small business owes money to the IRS. During her conversations with various agents, it became apparent that there is no cental database. Mind you, she was trying to pay and wanted to get a current amount owed.
She got bounced around to different agents. No single agent had access to all the data. Any high school kid or anyone with just a little experience with sequel programming could have pulled this info in minutes.
The fact is, the government databases, particularly the IRS are apparently archaic and compartmentalized. There is nothing challenging about consolidation by tax id or SS#. It just takes the will and money to do it.
It also takes the will to implement a system the will make a whole lot of union employees obsolete.