I pinged every IP address that wasn't reserved. The image is 8k by 8k and is re-encoded as an AVIF to be friendlier to mobile devices. Like every other survey done, it is using a Hilbert Curve to convert the linear address space to a contiguous 2d space. The hotter the colors (blue is coolest), the denser the ping responses were.
(If you are interested the full-resolution pyramidal-tiled TIFF can be downloaded and viewed in QuPath on desktop. I've also compressed the ping response data into its own format down to about 150 MB. PM me for a link)
Here is a 2006 survey to compare.
Some observations: Big Tech (USA) is in the top left. US government allocations, for the most part, did not respond to any pings. And maybe you didn't realize this before, but Multicast (Class D) & Class E consume a whopping 12% of the IPv4 range.
Nice! Now do the same for ipv6 :p
Any way to determine/estimate if the black squares are unused blocks ("give them back!") or just not responding to pings?
No. Any proper firewall is going to act as if there nothing there.
This was a ping scan, so it wasn't probing any TCP/UDP ports (see shodan.io). I suppose you could use ICMP control messages (Destination Unreachable) to determine if something was unused, but that assumes the other side is being friendly.