Have you checked for other Breeze styled cursor themes in Settings - Cursors - Get New

I see several Breeze variants, including one based on Plasma 5 (by tnaker)
Will that suit your needs?

I am aware of other themes and of themes that keep the old cursor, yes. I'm using https://www.pling.com/p/2137379 myself.

Since there's a theme that meets your needs, what would justify the work to add the old Breeze cursors back to Plasma?

I'm asking to change the shape back, not to bring the old cursors back.

* There was no technical reason to change the cursor shape, I'd assume there's no technical reason not to change it back, so if someone puts in the work to change it I'd hope it wouldn't get rejected
* The current cursor shape does not show originality
* Older cursors do not support SVG

To clarify, what I mean by not showing originality is that, if you compare a rectangle selection of the area around the cursor for Windows, GNOME, the new Breeze cursor, and the old Breeze cursor, the cursor that would be the most original in terms of shape would likely be the old Plasma one.

As there is no technical issue here, it basically amounts to "the old one was more original." Fair enough, but I don't see that as a compelling reason to go back to what we had before, considering that the new design has generally been well-received. In the absence of evidence that going back to the original shape would be a net strategic benefit to KDE in general, I think we'll have to keep this as it is.