I have 'Hide related VST2 plugins' set, and I have MeldaProduction VST2 and VST3 plugins installed.
Cakewalk 2019.03 shows both plugins in the plugin list. Is there something else needed to hide them?
For VST3 migration options to work, the plug-in manufacturer needs to add the appropriate code to their plug-ins
Here are a couple of blog posts about the feature
http://www.noelborthwick.com/2013/09/28/developer-notes-sonar-x3-vst-enhancements/
http://www.noelborthwick.com/2014/01/22/developer-notes-sonar-x3-vst3-internals/