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Please point me to the international law that says introduceyourself is only for posts from people saying hello here I am but not about introducing somewhere else or what new people should (not) do etc.

There's no law, but then Steemit has very few rules. It's up to the community to decide what is acceptable and the consensus on this tag has been established. Maybe someone has put together a list of other such tags. steemit-help is commonly used for tips