Sent one? Like using the USPS?

To which mailing address?
If the government is going to find these people they are going to have to get ICE to investigate and track them down, as they've disappeared into the interior of the country.
You seem to be firmly committed to the talking point that this process is what each migrant goes through. I think that's unrealistic and overly naive.
There has been reporting to the contrary:
Migrants who illegally come across the U.S.-Mexico border and are apprehended by Border Patrol are not being tested for the coronavirus before being released from custody and allowed to travel across the United States, the Washington Examiner has learned.
www.yahoo.com
The numbers are a sign of just how overwhelmed some sectors of the U.S.-Mexico border continue to be.
www.axios.com
Reporting above asserts many are being released without any NTA.
The immigration courts have had years of backlog for decades, it's part of the problem which ends up their being released into the interior of the country, only to disappear into the 'shadows'. This has been the situation very much even before Trump's admin, your deflection away from reality and history noted.