If I want a Unity feature that is not released in 5.6 or before, do I have to subscribe?

Symptoms

• I am a Unity 5.6 perpetual license user, but there are some features in Unity 2017.x versions that I would like to use.
• There were some features that were on the roadmap released in Unity 2017.x that I would like to access

Cause

Sometimes fixes or new features can take a little longer to implement than expected. Unfortunately, some in some circumstances they need to be pushed back to a later release. The reason for this is to make sure the content for these features are operational and consistent.

Resolution

Some features are only included in Unity 2017.x versions and you need to subscribe to receive all current updates. 

The end user license agreement (EULA) of Unity 5.x perpetual products stated that the license stopped receiving new features in March 2017.