Unsplash “Photo of the Day” is archived here: Photo of the Day (Archive) | 100+ best free wallpaper, background, outdoor and grey photos on Unsplash
This is the remote source that KDE pulls from, I believe.
(You might have to wait an extra day or so to see the most recent ones make it to the archive.)
The wallpaper is usually replaced by the “POTD Plugin” in here:
$HOME/.cache/plasma_engine_potd/
This applies to any source for the POTD Plugin, including Bing, for example.