P:(Masking): Allow Paste of Mask created in Photoshop to Lightroom

This is incorrect. Your logic only applies to the interface side of things, not to the internal rendering of the image. 

When you create a luminance mask, internally a bitmap is rendered that matches to the resolution of the current image and its luminance values. If the luminance values or the image changes, the bitmap gets recalculated. There is no existing method that exists to apply a luminance mask to a photo (which is a bitmap) without producing a bitmap. This is true even if you create the mask in some other format, such as a vector description; in order to understand what to do with the components in the image (each representing a pixel), a bitmap is produced (an injection s created that maps one source location to each individual destination location). 

You are conflating the luminance mask settings (equivalent to blend-if) with the luminance mask that this produces. Of course if you move the settings, you get a mask on the new image. But the desired result here is to move the mask itself (not the configuration to create the mask). 

People seem to think "parametric" means that magic happens. This is not the case. Lightroom is still a bitmap image editor. "Parametric" means that it describes the edits in a non-destructive way. 

You are incorrect that the brushes / masks are resolution independent. Masks are described in the XMP files as a series of "dabs", where each dab is a brush stamp of a specified size. The XMP also includes the ID for the computed mask value, but I have not been able to find the corresponding data in the database. 

[Abuse removed by moderator] The internals pipeline that renders the image necessarily has to create a computed bitmap in order to apply the mask. 

[ Note from moderator: forum guidelines are to be kind and respectful to others. ]

Perhaps if it helps to clarify this, we might use the example of an adjustment layer in Photoshop, which has been given certain Blend If settings to restrict its action according to luminance values in the layer stack below.

And then consider what happens, if you copy such an adjustment layer from one PS document, into another. You will not get the same 'practical' results from Blend If, unless it is being given the exact same input to operate with. That is part and parcel, of embodying the same 'logical' result.  

That kind of entity and that kind of logical operation, is probably the closest PS analogy, to a LrC local adjustment using a luminance mask. There is no mask bitmap involved, and nothing seen in the Channels palette. Just a layer that has been given dynamic rules, making it act in a certain selective way.

Extending this, something that there is no analogy for in PS: a Subject selection is merely an instruction of the same kind. The practical fruition of which must equally await the dynamic application of that instruction, onto this new image's own content. The requirement to approve recalculation is different, but harking back to the BlendIf example, if THAT did not recalculate itself continually and automatically to whatever is below, we would be dissatisfied with that.

In PS when we want a static mask we do that, and when we want a dynamic mask we do that. LrC / ACR is IMO somewhat ahead of PS layers in what you can do dynamically / parametrically and how efficiently; its focus is parametric and even, its brushed masks are resolution independent, because that is how it makes most sense to navigate its particular task landscape.

PS is way ahead of LrC when it comes to static bitmap masks and what you can do with those. That's clearly a very different task landscape being navigated by that. Hence seeking to map or transfer the tactics and solutions from one onto the other, is IMO going to be prove as pointless a quest, as it would be trying to mount oars onto a motorbike.

I don't know what you think you are demonstrating by configuring a luminance mask and then pasting it (which then recompultes) vs an unconfigured luminance mask. 

If you configured the second one the same way as the first they would look identical.

Here's the result of copying a luminance range mask from the left photo to the right photo:

Here's the result of creating a new luminance range mask on the right photo:

Can you explain the difference between pasting an existing luminance mask and creating a new luminance mask?

"Copying and pasting a Luminance mask results in the mask being recomputed on the new image (making copy/paste perfectly useless, since this is what creating a new mask does)."

Can you explain more why you think the current behavior isn't useful?  

"In the case of the automask, there should be an option to apply without recomputing"

Please submit that as a feature request:

https://community.adobe.com/t5/lightroom-classic-ideas/how-do-i-write-a-feature-request/idi-p/12386378

While waiting for Adobe to implement it, you could use the Copy Settings plugin to copy original Select Subject/Sky masks (without recomputing).

"Lightroom has neglected to allow masks from files"

As you've observed, that's currently not implemented by LR. If you want Adobe to implement it, I suggest you post a feature request in the Ideas section:

https://community.adobe.com/t5/lightroom-classic-ideas/how-do-i-write-a-feature-request/idi-p/12386378

My 'simplistic' understanding is that LrC is a 'Parametric' editor, Ps is a 'Pixel/raster' editor - they are totally different editing modalities.

I see reports of LrC masks being 'exported' to pixel based files to use as a Ps mask, but like RGB files cannot be reverted to raw data files, I would expect problems trying to convert Ps masks to work in a parametric editor.

Also interested to see a reply from 'one who knows'.