PaulArchivist

It tends more to be things which haven't been mapped yet rather than inaccuracies as such. Most mappers have concentrated on mapping their local town or city rather than national borders. See Kiev (https://www.openhistoricalmap.org/#map=16/50.4633/30.5218&la...) and London (https://www.openhistoricalmap.org/#map=15/51.5163/-0.0900&la...) for example. While the project may have been going a while, it was only last year that the time slider was introduced which made it properly functional as a historical map. At the moment there are still few mappers (including myself) but I get the impression that OHM is now beginning to develop more quickly!

Yes any feature as far as I'm aware can have a start date and end date so can show the changes over time. As can be seen in London (https://www.openhistoricalmap.org/#map=16/51.5108/-0.1086&la...) where the shape of the river changed when the Embankment was built in 1869. As far as I'm aware there is no reason why changing coastlines couldn't be mapped, though it is a lot of work to draw in a long length of coastline.

Yes it is and yes you can! There is more information here: https://wiki.openstreetmap.org/wiki/Open_Historical_Map