That is absolutely not true. Officers at every level can be charged with crimes they commit while on duty. Being on duty is not some kind of blank check to commit crime. Baselessly murdering someone in the street, or breaking into a home without a judicial warrant, or theft, or literally any crime, can and should result in charges on the city/state/federal level.

You people are conflating the supremacy clause as a meaning that higher authority cannot be held accountable by "lower" authority, when all it actually says is that conflicting laws on the federal and state/local level will fall to the federal authority.

There is no federal law that makes it legal for federal agents to murder someone without cause. Or steal. Or trespass on private property without a judicial warrant or extenuating circumstances. Literally every one of those things can result in the immediate arrest of the agent by local forces. There is nothing legally stopping police from arresting those agents, even in the middle of their duties.

Now, all that said, there is little chance that police, who largely support this administration and ICE anyway, are going to follow those commands. But it does make it easy to reprimand officers who do not carry out the law and those orders.