Celtic culture and Germanic culture:
By way of comparison, the Celts appear to have begun in the La Tene region of Europe, while the Germanic tribes appear to have originated with the Goths north of the Black Sea. (Mallory, 84) We can generally date
the Germanic and Celtic genesis as linguistic-cultural groups to about 500 BC. (Mallory, 87)
We find the classic three functions of priest, warrior, and producers here, as we do in
Gaul, and this culture is one of those that is often cited as good evidence of
tripartation. We also find a heavy amount of contact with the Celts, most likely due to the permeable border that
was the Rhine river. As examples, we can see that words for concrete concepts such as iron and lead were borrowed from Celtic, as were words for abstract concepts like slave and master. (Mallory, 113) The borrowing of the concrete words indicates that contact with the Celts brought both these metals into their lives, and the borrowing of abstract words indicates that there was something about the way that the Germans thought about social interactions which was changed by contact.
There is not much to contrast between the Celts and the Germans at this early stage, particularly because the Germans and Celts are so similar as to be often mistaken for one another (scholars still debate to this day whether Tacitus was writing about Germans or Celts in the
Germania). The border created by the Rhine is a rather artificial construct, as there was probably little actual difference between the tribes on both sides.
It is widely believed by scholars that even linguistically, the Rhine was a
permeable boundary, and we are likely to find people on both sides who can
speak and understand the languages of the other.
We do have a retention of IE divisions into a tripartite system in Iceland up until at least 1000 AD, when Christianity became the official religion. I do not see a point where IE status could be said to be "lost" by the Germanic peoples.