I think the perception is the much bigger story here than the actual change in advice by German authorities. Traveling to the US (and other countries as well) always had the chance to trigger some overzealous enforcement and/or arbitrary rejection or detainment.
But the current US government is very strongly ramping up the rethoric and actions in that area, so the risk increased and will very likely increase further. That creates an enormous amount of uncertainty and this uncertainty will have effects. For some people (white, from western countries) this is probably still a very low risk, but anyone that fits the target profile of the new government is in much more danger.
I don't want to downplay this by pointing to perception, but in the end the perception is much more likely to affect the behaviour of visitors than the murky real change. Even at a low chance, people quite justifiably don't want to risk geting locked up in a cold cell for an arbitrary amount of time. And the perception matters much more here than the quantifiable risk of getting detained.
But the current US government is very strongly ramping up the rethoric and actions in that area, so the risk increased and will very likely increase further. That creates an enormous amount of uncertainty and this uncertainty will have effects. For some people (white, from western countries) this is probably still a very low risk, but anyone that fits the target profile of the new government is in much more danger.
I don't want to downplay this by pointing to perception, but in the end the perception is much more likely to affect the behaviour of visitors than the murky real change. Even at a low chance, people quite justifiably don't want to risk geting locked up in a cold cell for an arbitrary amount of time. And the perception matters much more here than the quantifiable risk of getting detained.