Ext Plugins have been reintroduced for DXP version Fix Pack 8+ and will be deployable in Liferay Portal 7.0 GA4. We should briefly document this and explain the risks and that it's only recommended for cases where no extension points (using OSGi modules) exist. It should also be highlighted that Ext plugins are deprecated.
They only work from the Plugins SDK and work the same as in previous versions. For reference, the 6.2 docs on Ext plugins can be found here.