As far as I know, deleting files from a camera card AT IMPORT has never been a feature of Lightroom.
"Deleting" is perhaps a slightly strange way to describe the "Move" import option. As you say: never available from a camera card, but it has been available for a standard disk volume till now.
As an advocate of the "handle only once, but if you can, get someone else to do it" school of filing management - I personally have no place for Move in my ongoing workflow.
(Each new import comes off camera card according to an automated date based filing scheme implemented by Lightroom - programmatically. My entire involvement is: ensure the right (user) import preset is re-initialised; type in some new batch keywords to suit; click Import. Then leave them be; and do everything further from inside the Catalog, virtually.
So I haven't even had to think about folders and their contents, so far as LR source files are concerned, for many years. Time and attention invested, pilot errors committed: substantially, zero.
That said, it was certainly very helpful in the initial transition from a prior human-filed scheme, to be able to have LR Import those images using Move instead of Add, implementing the precise same filing scheme that was thereafter used going forward, with new images from camera card, using Copy.
In my opinion, it might even be more helpful to have a similar physical Move option inside LR, as an alternative to doing this during Import. Something which could be applied programmatically onto images which had already been imported - whether of long standing, or else freshly so. An extension of the current Rename tool, if you will.
I download all my files from my camera to my main operating system HD. Then I import them into Lightroom and use the move tool to move them to the HD I use for storing all my images and at the same time I send a copy to my backup HD. I upload my images to my main HD because it is synced my time machine HD and therefore protected from loss.
Just a thought: if you used Lightroom to import your images in the first instance, with Copy, directly into a location in the main HD then not only would they be immediately protected (I presume) by Time Machine; they'd also be immediately available for you to start editing / triage (and for LR to start building previews, if set to do so). No waiting for other operations to happen first.
As an option in Import, you can have LR copy a second file into a chosen backup location - immediately. There's a simple date-based or source-folder-based filing scheme you can select, for that - purely a last-resort option, in which case chronology of Import and Backup will probably be the more relevant.
So at that point there are up to 3 copies in existence: 1 still on the camera card, 1 filed into whatever location, filing structure and naming you have specified in the import Destination panel, optionally 1 further belt-and-braces copy in a backup location.
At your leisure, having confirmed a successful import and generation of previews, the copy still on the camera card can be safely wiped (e.g. format and re-use the card).
Also at your leisure, you can drag (within LR's Folders panel) the entire imported folder over into a corresponding structure within whatever different drive volume may be appropriate; that's if you prefer to use a 2-step rather than 1-step method for other reasons. Say: from a 2015 folder in the one volume, into a 2015 folder in the other volume. Doing this does not disrupt or invalidate any management or editing of these imported images by LR.
However. As a Mac ignoramus, I have to ask what may be a naïve question: could Time Machine not also cover that secondary drive, where you actually want your images to be? If so - could you not have simply sent the images straight there, when copying them from the camera card in the first place? (Or - if not - is another way to back THAT up, not still required?)
[The reason I mention this: a LR setup has two discrete backup requirements which are on-going: one for the 'active' Catalog, the other for the 'active' images location.
The latter includes not only a copy of the unchanging out-of-camera image data, but also any updated metadata / updated DNG preview which has been subsequently written to that.
Also this is the sole location of any derived files such as TIFFs or PSDs, generated in the course of external editing from LR.
So it's THAT location which one would normally want to protect with a backup regime; and one might make Catalog backups there too, so as to be included in the same protection.]
So why do you need to delete the files on the card? Your camera manufacturer almost certainly recommends formatting the card after downloading files, not deleting them using a computer.
The OP was not AFAICT talking about camera card, but about another intermediate location. And Import / Move used to work from that.
Moving them from there into the final location at import into LR, instead of Copying, saves the trouble of then deleting these intermediates.
Personally I'd leave out those intermediates altogether, and save even more trouble . I'd be putting LR in charge of all the physical filing, end to end, direct from camera card into a "black box" image store (fully LR managed set of folders).
Then again: I personally don't happen to enjoy painstakingly and time-consumingly micromanaging this kind of stuff, although many people apparently must, or at least, see a need.
It's surely a wrench to let go of the familiar, byzantine, ingenious working schemes which more literal file- and folder-centric ways of working have previously required. Which we are so proud of - whose awkwardnesses we have perhaps stopped noticing - which have become, over time, for us, simply: How It Is Done.