Admittedly, it is not enforced often enough and hard enough, but in general this falls under criminal organisations law.
German Criminal Code §129
Whoever supports such an organisation or recruits members or supporters for such an organisation incurs a penalty of imprisonment for a term not exceeding three years or a fine.
Germany bans certain symbols, including the Nazi swastika, the Communist hammer and sickle, and the Hamas flag.
But it does not otherwise ban support of those organizations/ideologies.
Of course they are banned. There is a long list with organisations and groups that were banned since 1951. https://de.wikipedia.org/wiki/Liste_in_Deutschland_verbotener_rechtsextremer_Organisationen
Not only symbols are prohibited. Extreme-Right / Facist / Nazis writings, music, images, videos, etc. are also banned.
Holocaust denial is forbidden. Historical relativism in relation to the Second World War is prohibited. etc.
Yes, the organizations themselves are banned.
Expressing support for them is not banned, unless accompanied by those symbols.
Admittedly, it is not enforced often enough and hard enough, but in general this falls under criminal organisations law.
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We’re talking about twitter as a company and platform, not twitter users.
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The hammer and sickle is not prohibited at all lol, or else the DKP would be banned