Looks to me like these researchers may have a bead on that data: https://armedea.wordpress.com/2014/10/06/quantifying-seismic-effects-on-european-populations-and-economic-activities-during-the-middle-ages/
I suspect they are using Maddison Project data:
https://www.rug.nl/ggdc/historicaldevelopment/maddison/releases/maddison-project-database-2020,
which appears to use modern political boundaries throughout. But
it only has century-mark data, so 1500 or 1400...
This brought up memories of McEvedy and Jones' 1970s atlas:
https://www.scribd.com/document/340967677/Atlas-of-world-population-history-b-pdf,
which I loved as a sprog...
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