It seems like this is adding some vectors on a gridded base map. But OSM has every object in a vector form, also coastlines etc. So I don't think there is a technical reason why coastlines would have to be fixed for a historical map. On the timescale of ages and millennia, rivers in delta planes are also usually not very static.
Well... coastlines are handled in a deep, dark, arcane part of OSM planet and data processing, but you're right. Rivers, on the other hand should be straightforward to change. We'll have some cleaned up examples in the near future.
Not only deltas, any river meandering over any plain will change its course pretty "quickly" (which creates problems if such a river is defined as the border between two countries BTW)
