Mmhhh, is it so strong?

I guess what is void is that specific clause. Otherwise, any vendor could include a clause saying "you relinquish some rights" just in the middle of the text and, after the fact, claim that the contract was void and require you to return an item (say, a car).

So, I guess you mean that specific clause?

In German law that would, as a default, actually make the whole contract void, yes. Many contracts therefore contain a clause that hedges against that situation (i.e. something like "if one or more of these items is found void, the rest still applies")

Specifically, the idea of a voided clause voiding the entire contract is sometimes a feature -- severability is not always desired. A simplistic example would be a contract with two clauses, one specifying that A sends B widgets, and the other specifying that B sends A money. You would not want those to clauses to be severable.

The legal term for this is "severability clause": https://en.wikipedia.org/wiki/Severability ("salvatorische Klausel" in German)

I believe this is how it typicaly works. At least the few laws I've read (Finnish law) voids the offending clauses, not the entire contract.