Event Horizon Telescope Data Cleanup
This process is described in great detail in this paper. The abstract says:


  To assess the reliability of these results, we implemented a two-stage
  imaging procedure. In the first stage, four teams, each blind to the
  others' work, produced images of M87 using both an established method
  (CLEAN) and a newer technique (regularized maximum likelihood). This
  stage allowed us to avoid shared human bias and to assess common
  features among independent reconstructions.


They then generated a large number of "plausible" images, reconstructed the radio signals they would have given and ran them through the image analysis pipeline to see if it showed any biases.


  In the second stage, we reconstructed synthetic data from a large survey of imaging parameters and then compared the results with the corresponding ground truth images. This stage allowed us to select parameters objectively to use when reconstructing images of M87. Across all tests in both stages, the ring diameter and asymmetry remained stable, insensitive to the choice of imaging technique.


Incidentally, it's not really appropriate to think of the rejected data as image data in the sense you would get from a camera. It is (almost) recordings of the exact electromagnetic waveforms received by the telescopes. The image arises from subtle correlations between these datasets. So most of what is thrown away (apart from noise) is information about the exact phase of the radio signal on a nanosecond-by-nanosecond basis, which is not really interesting.