The Enhanced Contrast Dark and Enhanced Contrast Light basemaps are now out of beta. These are designed to offer higher contrast for the visually impaired. For details, see:
https://www.esri.com/arcgis-blog/products/arcgis-living-atlas/mapping/working-with-enhanced-contrast...
For all related Living Atlas high contrast content, see:
https://livingatlas.arcgis.com/en/browse/?q=%22Enhanced%20Contrast%20Map%22#d=2&q=%22Enhanced%20Cont...
These two are now part of the Esri default basemap gallery if your organization region is set to United States (Settings > General > Organization Defaults > Region).
Those organizations using a different region can still use these basemaps using a custom basemap gallery, or start with the high contrast basemap after signing in at the Living Atlas website.
How to make a custom basemap gallery using Living Atlas basemaps:
https://www.esri.com/arcgis-blog/products/arcgis-online/mapping/living-atlas-custom-basemap-gallery/
