goproxy states that

GET $GOPROXY//@v/list returns a list of all known versions of the given module, one per line.

but also, a few lines later, that

To avoid problems when serving from case-sensitive file systems, the module and version elements are case-encoded

That seems to imply that because version is case-encoded in file paths and urls, it is also encoded in lists. However, “seems to imply” is not the same thing as a clear specification, and
rewriteVersionList for example, does not seem to perform any encoding check before writing indexes.

(practically we will need to force lower-casing in the versions we process for other reasons, but the go version spec, unfortunately, is not so strict).

That's one of the many little things that make an official go mod index desirable (issue #31303)