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

How will this affect Nearmap users?

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:

The underlying connection was closed: An unexpected error occurred on a send.

The ArcGIS 

You can Test ArcGIS TLS 1.2 connections to ArcGIS Online