*** Somehow- in the middle of a mixing session- Splat Plugin Manager "forgot" where the VST3 plugs were.  C/programfiles/commonfiles/vst3.  Once those paths were reset in the plug manager, I was back in biz.***
 
Out of NOWHERE, Splat doesn't find most Waves plugs when opening an existing project!?
 
If I search for them, they're in the menu, but tellingly, they are not in the "by category" plugin menu.  It's an issue, I think, only affecting VST3 plugs.  
 
In the plugin manager, there are very few Waves plugs and my total count of plugins seems WAY down.   Sadly, even a complete reinstall of my Waves plugins has done nothing to remedy the problem.
 
Help, please.

Did you check your licenses in Waves Central? Is your WUP (for any of your plugs) active? If so, ask Waves, their support is excellent.

Plugin browser has some nasty bugs. I also had problems similar to yours. I have zylions of plugins (tested lots of freeware things before  started to buy commercial ones) .If it is not issue related to licenses try 2 things:
 
1.use sort by manufacturer.
2. If it also won't help try to go to manage layout then in plugin categories (VST2 or VST3) select needed plugs in registered plugs and add to layout and save as new layout. Then select your saved plugins layout.
If you cannot find in registered  use scan option (in preferences you can use 'strength' of scan - use for this time manual scan).
 
Of course I assume you didn't change VST scan  paths.

Reset and re-scan? Might be necessary after a re-install. 
 
I had the same problem once when the USB dongle with the licenses didn't connect to the port. Re-starting the computer allowed it to be discovered. Probably not the same cause, but that did show the same symptoms.