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Yes, that's a good point.

Is there a good guide to how Steem consensus and witness shuffling / selection works?

Found Steem Witness Scheduling Algorithm by @dantheman. Not sure what has changed in the last 4 years since PoW mining was removed.

So did @dantheman foresee this when picking the one from the non-top-20 witnesses?

Soft consensus is not intended to be as air-tight as hard consensus. So it's not surprising this is possible. I believe the fact that it is possible means that DPoS is not as broken as people say it is.

Meaning: Steem is censorship resistant. Soft consensus censorship is not absolute censorship.

Also a merged pull request from 2016 titled New witness scheduling algorithm. By @arhag