As is shown in the pictures below. Perhaps Mojang considers this release as 1.13-pre11😛.
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kumasasa
At least you did search at all...
MC-134129
Wrong warning message in 1.13 release before opening a world
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kumasasa
Fu Chunhao
(Unassigned)
Unconfirmed
Minecraft 1.13
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It was my fault that I didn't search very carefully and try every possible key word before creating this issue... Please mark this as duplicated.