Heightmap Data
The heightmap data comes from NextZen, which was set up when Mapzen shut down.
It is not 100% clear what the usage rights for the heightmaps are now. This was the "rights" page that appeared on the Mapzen site.
This was their attribution guidance, specifically.
If in doubt, the following snippet should be included on your project:
© Mapzen, OpenStreetMap, and others.
© <a href="https://www.mapzen.com/rights">Mapzen</a>, <a href="https://openstreetmap.org/copyright">OpenStreetMap</a> , and <a href="https://www.mapzen.com/rights/#services-and-data-sources">others</a>.Other Maps
All other maps should have a link to the copyright information on the bottom left of the map display.
This data should be used for the "Export Albedo" type map data.
Most map providers can be found on the leaflet providers page.