I'm currently doing a research project analyzing how cities evolve over time and was wondering if there were any good online databases for finding historical building footprints. I've been looking around for a while now but couldn't find what I was looking for.
http://re.city is an effort to organize historical building footprint data. Please feel free to contribute your findings.
In the case of Denver, building footprints were only collected in earnest in 2017 by the Regional Council of Gov'ts project. Before that, it was just haphazardly available in OpenStreetMap. Parcel data is sporadically available from the City and County, but nothing publicly available. I think that every city would be different, but doubt there would be much in terms of footprints available.