Well, but that forks only the Steemit website, not the Steem cryptocurrency behind it. The latter would be, as far as I know, a license infringement because Steem is not open source (it's "source-available" or "shared source", but it forbids you to create a clone with a new genesis block).
I have an idea of a re-implementation of a similar (but a bit different) content monetization scheme, based on an already existing coin. But it will take some time

Steemd the backend of the blockchain was made opensource long ago.
Considering golos is a steem clone and there is another as well that used steem code.