I made a bug report and it looks like upstream is not interested in reverting the cursor shape. If someone were to make an actual MR changing the shape back I don’t see a technical reason why it would be rejected, but I currently lack the skills to do so. Honestly, this is worth a bounty in Sponsored Work.
Until there’s someone who’d do the work of changing the cursors back to the original shape (but keep the colors and contrast changes), I found a cursor on the KDE Store that does the job:
(looks like @schwarzerkater already found it 
)
As mentioned above, if someone wants the shape and colors back, there is this:
Although no one complained about the color changes at all.
Unfortunately neither cursor theme supports SVG, so the Shake Cursor effect becomes obviously pixelated.
I also found a bug in the SDDM integration that prevents you from using custom cursors installed userwide. So setting the cursor and then going to System Settings -> Colors & Themes -> Login Screen (SDDM) and clicking on Apply Plasma Settings... doesn’t actually work, and instead makes SDDM use some sort of fallback cursor that looks like Adwaita (it probably is).
To work around this, you can extract the cursor theme and then copy its folder to /usr/share/icons, set the cursor theme, then Apply Plasma Settings and it should work.