Insightful post as always, Jim.
The 2020 proposal would reduce the "jump" limit to 1.5 miles. Currently, urban areas may jump across non-residential areas for a distance of 2.5 miles, to reflect discontinuous development along highways which are part of the urban population.
Before 2000, the jump limit was 1.5 miles, even though urban areas were defined differently. The 2.5 mile limit was adopted perhaps out of concern that an automated process might produce bunches of urban clusters that were treated as independent of the nearby city even though they functionally were not. In 2010, the Census Bureau considered a return to the 1.5 mile threshold, but the response was equivocal. It appears that the Census Bureau is simply going back to what they wanted to do in 2010.
If this rule were to get established, would it mean that we'd see some Urbanized area mergers (and thus metro area mergers as well), such as for instance San Francisco-San Jose or Los Angeles-Riverside? Those two urbanized areas already share borders with each other, but from what I know, their residential area borders are too small to classify them together.