The mime type used by Spring Web Reactive for streaming json does not seem to be correct. The mime type "application/stream+json" is for an obsolete standard for activity streams:
It has been superseded by the standard:
with mime type "application/activity+json".
However, an activity stream is not the same as streaming json, so neither of these mime types should be used.
I was unable to find any standard mime type for json streaming, so I'm not sure what the resolution should be. The mime type should not be in the standards tree (since it is not a standard mime type), and the "x-" tree seems to be deprecated (since 1996) (see https://en.wikipedia.org/wiki/Media_type).
But I'm not sure that using a deprecated media type in the standards tree which is not officially registered, was invented for a different purpose, and has been superseded, is the way to go.