mark this project as dead

[matkoniecz]

It may be a good idea to explicitly describe this project as dead in readme

[jerryhall]

How can we keep it alive. This is an amazing collection that we could add to?

[pgrandin]

It would be easy to fork it for updates, but in this case @twain47 should probably update a readme to point to the new repo.

[PanderMusubi]

One of the forks in https://github.com/twain47/Open-SVG-Map-Icons/network should be designated to become the new upstream and merge other forks into that, or ownership of this repo should be transferred to one of the most active fork maintainers. Then also update http://www.sjjb.co.uk/mapicons/contactsheet and OpenStreetMap. I hope these icons keep being maintained. See also PanderMusubi/inkscape-open-symbols#39

[PanderMusubi]

Please follow the work done at PanderMusubi/inkscape-open-symbols#39