We are aware users running PaperMC are seeing this warning message. Please rest assured this is nothing to worry about. We have already planned to update to Java 11 before the Minecraft 1.17 release - this is simply a early warning to give enough time for preparation. The reason we are not updating before the release is due to the vast amount of plugins that are not readily updated to Java 11 yet - and therefore would break/make them not compatible. We anticipate with the PaperMC warning, that plugin authors will have the needed updated out before this.
If you have any other questions regarding this, please feel free to ask our Support team via Support ticket or LiveChat (when available) and we will be more than happy to help!
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