I've been active in the OpenHistoricalMap (OHM) project for quite a while now. We've had our ups and downs, but are busy recovering from a major server outage and things are looking better and better for the future.
My early work has been in mapping historic race tracks in North America. There was some complexity in locating them and finding maps and or imagery to allow for mapping them in OHM but ultimately, you dropped them in place and set the start and end dates for their usage, and it was done.
Things have gotten a lot more complex recently. I've been working on mapping the history of the Capital District of New York, starting with Albany, with plans to radiate outwards and map Troy & Schenectady.. This has made things a little interesting. There is a relatively recent episode of Doctor Who in which all of history happens all at once. If you are going deep in time in an OHM project, the editing process consists of trying to map all of history all at once, and the more detail you map, the more challenging the editor becomes to work with.
There is a filter mechanism in JOSM (the OSM editor I use) that helps to some degree - I have a limited ability to ask for things that started or ended before or after certain years - but it could use some refinement. OHM's approach to dating could use some work as well; the Gregorian calendar stops in 1532, and older things may have some uncertainty about when they actually happened. For example, no one is entirely sure on when the northern end of the Albany Basin was filled. We know it was before 1910 and after 1892 but there's a bit of guesswork involved (and the answer may be in some forgotten records in the Albany city archives.)
The other project I've started on is the Antietam Battlefield from 1862. But that's a topic for another posting.