Urban/rural in IPUMS ACS post-1990

Does anyone know why the IPUMS ACS urban/rural variable (URBAN) isn’t available post-1990? It looks like census.gov still reports summary statistics on urban vs. rural populations post-'90. Is it a comparability issue, or did census.gov stop putting it in the publicly available microdata?

A few notes should be helpful. First, urban/rural status is one of the most difficult geographic variables to construct over time. The Description Tab for the URBAN variable explains these issues in more detail. Second, it looks like the Census Bureau currently defines urban areas as follows: Urbanized Areas (UAs) of 50,000 or more people and Urban Clusters (UCs) of at least 2,500 and less than 50,000 people. In public use data, however, the lowest level geographic identifying variable is the PUMA, which are defined by areas of at least 100,000 people. So, any current rural/urban designation likely uses restricted access data. Finally, note that even in the 1990 samples available on IPUMS USA, the urban/rural status is computed by IPUMS and not given by the Census Bureau. Specifically, the URBAN variable is derived from the universe of farm status (FARM). Personally, I’d suggest using a more explicitly defined concept, such as metropolitan area (METRO), which is also has much more availability.