The bug

On the world selection screen, when clicking "Edit" and then selecting "Optimize World", the following text (translation key optimizeWorld.confirm.description) is shown:

This will attempt to optimize your world by making sure all data is stored in the most recent game format. This can take a very long time, depending on your world. Once done, your world may play faster but will no longer be compatible with older versions of the game. Are you sure you wish to proceed?

The highlighted text is rather misleading. This text appears always, even if the world has been opened in the current version already and can therefore be misunderstood that downgrading is normally supported (which it is not).

Additionally it does not indicate clearly enough that optimizing a world which has not been upgraded yet, potentially corrupts the world / deletes information (see MC-181278, which might be the intended meaning of the sentence).