After immigrants are detained by ICE, there are probably records somewhere of who has an attorney, and who is calling family members or an attorney.
For the immigrants who aren't calling any family or lawyers, the poorest of the poor, people with little to no connection to anyone, who may not even say which country they are from, if the administration were actually trying to kill as many disconnected immigrants as possible in secret, would society know?
For example, if the administration was composed of all types of conservative people, but 30 percent were secretly Nazis or wanted racial genocide, could there be a campaign to not just remove, but kill, immigrants, and the public doesn't find out?
Possible, but highly unlikely.
For starters, if that many were killed, I think it'd be really hard to keep a lid on it these days.
Secondly, I've done a lot of reading on the atrocities of WW2, and two immediate constraints come to mind:
Bodies. While possible, disposing of that many bodies without anyone noticing will be really hard.
The gas chambers of ww2 were only planned during the Wannsee conference in 1942, long after the holocaust actually started. The gas chambers were built because the original plan was not sustainable: People killing other people at an industrial scale. No matter how vile a person is, the human mind was not built to deal with having to mow down swathes of humans with a gun. Even the worst SS troops largely couldn't deal with killing that many defenseless people, even if they were convinced it was "for the greater good". The amount of SS troops who ended up unable to continue due to the mental after effects is staggering.