Yes Kev, I mean 'layouts'. No, that's not what I'm trying to achieve. Thanks.
 
An update:
It has been confirmed to me by the infamous Mr. Wookie that he can replicate the problem.
"I can confirm, that although you can create you own plugin layouts in 8.5.3 you can not select them for use."
 
These are the instructions for replicating (if anyone's interested)
 -pretend that you have no PM layouts/presets in 8.5.3. so you need to make one to get to your plugs.
 -create a layout/preset in PM and put some plugs in it. Save it. Exit all.
 -Start 8.5.3. create audio track.
 -right click the fx bin in the audio track and select the new layout/preset you created.
 -right click again to pick a VST plugin
 -Are there any VST plugins available there?
No.
 
 

timidi
These are the instructions for replicating (if anyone's interested)

I was still on last month's SPLAT update, and couldn't replicate (on 8.5.3).  I just now updated to the latest SPLAT, and now I can replicate!

I did more testing and rollback'ing for SPLAT and have reached these conclusions:

• First, please note that all my testing was done only with 32-bit versions of both SPLAT and 8.5.3.
• The plug-in manager (PIM) is shared between SPLAT and 8.5.3.  Namely,  C:\Program Files (x86)\Cakewalk\Shared Utilities\PluginManager.exe
• The SPLAT update of 2017.01 (23.1.0.32) changed the PIM (to a modified date of 01/26/17).  This new version causes an incompatibility (as described above by timidi) with any new plugin layouts created in 8.5.3.  Older plugin layouts still work ok.
• Rolling SPLAT back to 2016.12 (22.12.0.12) changes the PIM to 12/04/16, which doesn't seem to have the problem.

(Aside: I wonder if perhaps SPLAT should start using its own version of PIM, so it won't affect 8.5.3?)

Thanks for chiming in tparker and the further diagnostics. Yea, it's broke.
Good to know it's OK at 12/04/16. 
 
"(Aside: I wonder if perhaps SPLAT should start using its own version of PIM, so it won't affect 8.5.3?)"
 
This is something I'm wondering and may start a new thread that answers the question:
Can Splat and 8.53 be installed separately so there is no interaction between the two? My main concern would be the registry and whether or not any Cake entries would default to what they are now (shared).
And then there's the command center which might not like something other than C:/program files/Cakewalk.    
(IE: C:/program files/Cakewalk splat.    C:/program files/Cakewalk 853).
etc.   

You're welcome.  And thanks to you for alerting us to this issue and persisting with it. 
 
It's kind of sad really (and scary) that updating SPLAT should break previous versions.

I like your idea of starting that new thread, too.

I think they get weary of my persistence around here.

I looked inside the PGL files created with the actual version of Plugin-Manager and older files. It looks like the Plugin-Manager now stores the clsidPlugMD5 for VST plugins, but before it used the clsidPlug (as listed in the registry under HKEY_CURRENT_USER\Software\Cakewalk Music Software\SONAR\Cakewalk VST X64\Inventory).

Good detective work!

ralf
I looked inside the PGL files created with the actual version of Plugin-Manager and older files. It looks like the Plugin-Manager now stores the clsidPlugMD5 for VST plugins, but before it used the clsidPlug (as listed in the registry under HKEY_CURRENT_USER\Software\Cakewalk Music Software\SONAR\Cakewalk VST X64\Inventory).

Good find. Thanks.
So, what I derive from this would be that install of Splat from 12/16 on would orphan the 8.53 PIM files? 
 
I'm guessing this would mean that Splat and 8.53 would then use separate isolated registry entries for their own plugin manager exe's.

in theory you could modify the pgl files yourself?