P: Support cataloging PSB files

Agreed!  With higher MP cameras becoming normal I'm hitting that 4gb limit more and more often when stitching even moderate to small sized panos.  I do all my cataloging/sorting with LR.  What's the point in using LR for that if it can't catalog/sort a file type I use often?

I've created a workaround in LR to at least get access to the PSB a little quicker than navigating through the computers OS:

Here's what I have been doing: Create a "PSB" Folder within LR. Then save your PSB file within that folder from within Photoshop. While LR won't see the actual PSB file, it will see the PSB folder, and if you right click on it and choose "Show In Finder" (on a Mac) it will open that folder from within the Mac OS, just double click on it and it will open in PS. This should work on windows as well.

Note: If you don't create the PSB folder in LR, then LR will not see that folder PERIOD. Even if you synchronize a specific set of folders. 

"Why can't Lightroom work with a flattened proxy"

That's exactly how LR works with multi-layer PSDs.  When you have the Maximize Compatibility option checked (required to work with LR), Photoshop saves a hidden "compatibility layer" in the PSD that's a flattened copy of the image, and LR reads that compatibility layer. LR chokes on PSDs that don't have it.

Agreed, its not like there hasn't been time to get this working, it's taken the photo industry 14 years to go from 12MP standard to 45MP standard, and that's if you totally disregard the Pro pro guys that have been shooting 60-120 for years, it basically makes Lightroom unusable for them for any sort of comp workflow. 
But the 65,000px limitation is besides the point. I can open a PSB in Lightroom if I flatten it first and save it as a compressed TIF in PS. Why can't Lightroom work with a flattened proxy so I don't have to maintain two different files and multiple saves every time I make a change to a PSB?

Maybe it's high time to rebuild and optimize the camera raw engine.

Increasing the pixel limits significantly might involve much more re-engineering in LR (e.g. throughout the Camera Raw engine).

heh. Many of my files are well over 512MP... Rodeon Pixplorer and D850 w/50mm panos... 

The amount of engineering effort for LR to support PSB is modest at most. The PSD and PSB formats are nearly identical, the only difference being that PSB uses 64-bit file offsets where PSD uses 32 bits. For my PSB Quick Look plugin, I modified ImageMagick's PSD module to read PSBs and only had to change less than a dozen or so lines.

LR already imposes maximum size limits on all photo types, 65,000 pixels on a side and no more than 512 megapixels, and it could impose those same limits on PSBs. (Many, if not most, people who want PSB support in LR need it because their layers cause the 2 GB/4 GB limits to be exceeded, not because they have too many pixels.)

RIGHT! They could easily read the file, create a proxy and catalog the proxy and only allow edits on that. Exports would be generated from the PSB. Not sure why they are not connecting the dots here. My computer has 128GB ram. I'm fine if they say "PSB requires 64+"

It is absurd that they are not dealing with this, as it is a major issue. I LOVE lightroom, but this issue is a major problem for many of us.