1. The entire legal system in Europe, and in Germany in particular, is based on the concept of "dignity". This is the foundational stone for every other concept in the legal context, also including the "free speech". You have to treat other people with dignity.

2. For every freedom, as it understood in Europe, holds true that "your freedom ends where freedom of other people begins". It means that your free speech is (and was) never allowed to attack other people personally (every kind of insults), professionally (e.g. reputational harm), as a member of specific group (any kind of sexism, racism, defamation, etc.)

Police and courts were not always strictly following this, due to various reasons, but it doesn't change the definition. And it is valid for online speech in exactly the same way as for any other kind of speech.

And actually, also in the US it was defined in the same way - it just wasn't written down, since it was a common understanding at that time - and manners, also covering the art of speaking to each other were actually much stricter back then compared to today.