The superflat preset "Overworld" is not updated and keeps the bedrock-stone-dirt-grass layout instead of the bedrock-deepslate-stone-dirt-grass layout.
Linked issues
duplicates 1
Comments 2

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We're tracking this issue in MC-215423, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as invalid. Please take a look at the parent ticket (MC-215423) and see if an explanation is provided there in the description of the ticket or in the comments for why this issue is invalid.
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Possibly a duplicate of MC-215423, which was resolved as Invalid due to it being a feature request.