Because of the changes done to support Qt for Python, previous plugins won't work anymore in Substance Designer.
In particular, please note the following:
Plugin loading and unloading
Plugins are now loaded when Substance Designer starts and are unloaded when the Designer exits.
As such, it's not necessary for plugins to inherit from 'sdplugins.Plugin' anymore.
For more information, check the Plugin basics section.
Creating user interface elements
Plugins don't need to define a 'sdplugins.PluginDesc' anymore.
Instead, plugins can use the new UI manager object and Qt for Python to create any user interface elements they need.
You can find small code samples in the Creating user interface elements section.
Replacing uses of Location Context
The 'SDLocationContext' class has been removed from the Python API.
Plugins can use the UI manager object to access the currently active graph and selection.
You can find some examples in the Accessing graphs and selections section.