Specifically for iZotope and Meldaproduction plugins, the folders/lists for these guys are long, and selecting the "Hide related VST2 plugins" doesn't seem to be working for me (I am seeing both regadless of how I cycle this setting around).
 
Because of this, it also makes inserting an Audio FX (other than searching for it in the browser, or if it happens to be one of the 5 most recently inserted) rather painful. When seaching via right-clicking the FX bin, then "Insert Audio FX"->Vendor->and the list you see takes up the whole height of the window, scrolling through a list that is 5+ windows high by clicking on the up/down arrows at the top/bottom is a bit extreme. A scrollbar would be nice, especially if linked to the mousewheel. Currently neither exist.
 
At least if only VST3s appeared, the lists would be half as big.

The option works as long as the plug-in manufacturer implements the feature see http://www.noelborthwick.com/cakewalk/2013/09/28/developer-notes-sonar-x3-vst-enhancements/
 
Last I checked iZotope and Melda Production do not implement the feature. As a result, I installed their VST2 plug-ins outside the SONAR/CbB scan path rather than exclude them using the plug-in manager.
 

is there a list of the devs that DO implement this?

Thanks for the confirmation Steve. I suspected (feared?) that might be the case. That article is pretty old and the only ones quoted were Celemony and Waves, so no idea who else it applies to at this time.
 
Is the required spec laid out where it could be passed on to the folks at iZotope and MeldaProduction? I didn't see a detail of "what is needed," but may have missed it.
 
 

http://www.noelborthwick.com/cakewalk/2014/01/22/developer-notes-sonar-x3-vst3-internals/
 

I figured that rather than wait for Vojtech and co. to implement it in their plug-ins, I'd just spend 10 minutes communing with the Plug-in Manager and exclude the Meldaproduction VST2's. I still keep VST2's around due to Mixcraft's sometimes difficult relations with VST3's, but if they exist in VST3 form, I don't need them in Cakewalk.