ESRI recently sent out a reminder that starting April 16, 2019, they will be updating ArcGIS Online to enforce the use of TLS version 1.2 only.  The update is likely to affect most ArcGIS software and your custom solutions.  If you have not updated and validated your system’s support for TLS 1.2 only, you may lose your ability to connect to ArcGIS Online.

If you use ArcGIS Pro to connect to ArcGIS Online, you are fine if you are using versions 2.0 to 2.3.  If you are using Pro 1.2 to 1.4.1, these versions are all retired and you are encouraged to upgrade to the recent version.

If you use ArcGIS Desktop to connect to ArcGIS Online, version 10.7, to be released in first half of 2019, will support TLS 1.2.  Version 10.6.1 and earlier will require a patch to enable TLS 1.2.  ESRI has provided validation web services so you can quickly verify that ArcGIS Desktop, third-party apps, and custom components will work when TLS 1.2 only is enforced.

For more info on this, see ESRI’s technical support article on TLS.  Also see the list of software products affected by this change.  To enable TLS 1.2 on ArcGIS Desktop, see this FAQ.  Validation web services can be found here.