hello speedrunners, lately I was playing out some category extension runs on Minecraft java and I had them rejected because "recreate is not allowed" so I'm wondering what this recreate rule

If you read the game's rules, then it says:

• You cannot use the recreate world button to generate worlds in versions 1.16 and above. For any version prior to 1.16, you may use the recreate world button.

And in the future, you should ask game specific questions in that game's forum or Discord. Or ask the game's moderator(s).