ArcGIS Online have transitioned to utilizing TLS 1.2 only on April 16 2019, and some clients/versions will experience a disruption of service if they do not follow the steps described in Esri's TLS_Guidance.
This Esri document also lists which clients require update/fixes to support TLS 1.2, including what Esri products and a list of older browsers.
Esri has also established a support page to help customers at https://support.esri.com/en/tls.
When you try add a layer from ArcGIS Online, and your application hasn't been updated, you may get an error similar to this one:
You can Test ArcGIS TLS 1.2 connections to ArcGIS Online