The Landsat GLS services and layers from imagery.arcgisonline.com were in Mature Support and set for retirement for over a year. As of April 16th, 2019, the services have been retired . Any users who have existing layers and web maps referencing the old ArcGIS Online GLS services will need to migrate to the currently supported layers and services. This includes users on Enterprise Portal versions 10.5 through 10.6.1.
All the same imagery is still available through our current Multispectral Landsat image layer, which includes both historic GLS imagery and current Landsat 8 imagery. We also provide a pre-filtered GLS only view, Landsat GLS Multispectral. For additional options, users can find other Landsat layers available in the ArcGIS Living Atlas of the World.