Update: it turns out the complete BAG dataset, including historical data, is also publicly available to download.
Dutch OSM user Sander_H has been very helpful by generating GeoJSON files for every placename in the Netherlands using this dataset. They can be found here. These files include both currently existing and removed buildings. For removed ones, the date on which they were marked as demolished in the BAG is set as end_date.
We could start importing using these GeoJSON files. However, they are not completely without flaws:
- There are some demolished buildings that have an unknown year of construction. These have all been tagged with start_date=9999. I’m not sure what’s best to do with these: not import them, as they can’t be correctly dated, or importing them with an additional fixme tag? In other cases the year of construction seems to be an approximation. Other sources sometimes have a more specific year (e.g. 1853 instead of 1850).
- There are also some buildings that overlap each other. See this screenshot for an example: (the unselected building has existed from 1882 till today)
This is probably the result of the building on the right being expanded at a certain time, such that it now contained the building on the left, which was thus marked as demolished. (Satellite imagery for comparison.)
It turns out things can get quite tricky with importing historical data 
So, what would be the best approach right now? I personally think first just uploading all currently existing buildings might end up to be the most useful thing. After that we could add the historical data from BAG, making sure to manually check for things like the two problems mentioned above. But let me know if there are other ideas! (Maybe based on experience with previous similar imports.)
(By the way, I’ve also been thinking about how to approach the source tagging of this import. There will certainly be situations were we want to change one tag of an imported building while keeping the others at what is in BAG. An example of this could be when we find the correct start_date of a building that was originally set to 9999. Geometry and end_date would then still be sourced by BAG. So splitting into start_date:source, end_date:source and geometry:source would be useful, I think. But we can discuss that more extensively later, for now the things I mentioned above are most important.)