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Do you know who it is who actually makes these types of decisions. Is it just the top 20 witnesses?

It depends what decision you mean. We can all decrease the value of a post with a downvote. But downvotes are not very commonly used for this. But they can...

No. I meant the bigger decisions like how votes are paid out. The split, the dust amount etc. etc.

I don't know that either.

I also suggested to configure a maximum reward for posts. I think would need to be changed in the blockchain. I don't know who decides these kind of changes.
Maybe @acidyo knows.