I'm dealing with the same situation, and have some new insights.
I submitted a bug on the web page of the documentation, and got some feedback.
The feedback stated some preview is/was going on. I requested access but got no response whatsoever.
The slack group also contains discussions about this topic - didn't you use its search functionality?
At least someone got access to the preview, he stated a new Data Transfer became available after being approved.
So, how do you deal with the situation?
For us it is obvious that the BigQuery team is not that much interested in this feature, otherwise they would have started documenting this, sharing their plans publicly, etc. Ask yourself, is the feature mission critical? Then you should look into alternatives. Note that some companies (FiveTran, Supermetrics) have similar offerings.
In our case their deals weren't suitable, now I'm back to coding again... with a nice deadline.