The change applies to our customers using ArcGIS Online and ArcGIS Enterprise. We especially ask customers with ArcGIS Enterprise 10.3 to be extra attentive and to perform the necessary checks.
From 10.4 Https Only is the default, but that can be changed by the Administrator. Here too we ask to be attentive and to carry out the necessary checks. More info can be found in following support document : https://support.esri.com/en/https_only