As of 2.6.x the WebSockets section in the reference guide is gone. This used to be https://docs.spring.io/spring-boot/docs/2.5.x/reference/htmlsingle/#features.websockets and we believe it was removed by accident when the reference guide was split in multiple files.
We should consider re-adding it with an anchor rewrite in the io section.
Comment From: snicoll
It turns out the content was still there but was missing an import.