To start off, let's take a look at old-style plugins. So what are they?
They are a way of adding extensions or functionality to open-source SoapUI.
They typically create/configure new SoapUI framework elements, for example,
Models,Actions,Events,Listeners, andFactories(more on those later).They are written in Java or compiled in Groovy.
They are packaged and deployed as JAR files.
There are currently six example plugins available for download, for example, Email TestStep, Groovy Console, Programmable Web, RAML, Runscope, and Swagger plugins.
The following are the reasons:
SoapUI pro/SoapUI NG has a newer plugin framework (see the Plugin Manager recipe for more information)
The new pro plugins are not backward compatible, and old-style plugins cannot be used with the pro plugin manager
However, old-style does not mean obsolete, since if you are an open source user, the old-style plugins are in fact current and work perfectly...