This resource is awesome. I wanted to have the smallest readable QR code for my printables pages footer [1], and I discovered there that it's possible to encode URLs in "Alphanumeric mode", version 1, using only uppercase characters, e.g., HTTPS://FOO.CC/ABCDEFGHIJ, so I got a three letter domain name and built the simplest URL shortener hosted on a CF worker.
The URL shortener only use uppercase letters for the path of the URL, the domain and protocol are not case sensitive. Modern phones can read such QR code easily and identify them as URL.
[1]: https://pdf.ahaprintables.com/pdf/preview/aha/zebra-puzzles-... (PDF)