For completeness re Arcgis see this https://github.com/Esri/arcgis-osm-editor/issues/104#issuecomment-157990157 long discussion (including its eventual resolution).
Has someone already tried to contact ESRI about permission to use the clarity layer? I’ve just pinged two of their employees (who had been very supportive when getting the “standard” ESRI worl imagery officially into iD/JOSM/etc.): https://github.com/osmlab/editor-layer-index/pull/358#issuecomment-359237579.
Thank you tyr_asd for following up, and a big thanks also to jgravois!
For clarity (pun intended), that URL for custom map tiles using World Imagery (Clarity) is:
https://clarity.maptiles.arcgis.com/arcgis/rest/services/World_Imagery/MapServer/tile/{zoom}/{y}/{x}