ArcGIS Online uses TLS to communicate with the rest of the ArcGIS Platform. Today, it can use a few different versions (1.0, 1.1 and 1.2). But from April 16th 2019, for security reasons, ArcGIS Online will be updated so that it only supports TLS version 1.2.

Read this blog to find out what you need to do to ensure that any client application that communicates with ArcGIS Online will continue to work.