Here's an example off the top of my mind:

https://github.com/go-gitea/gitea/blob/main/go.sum

I think you are reading that wrong, go.sum isn't a list of dependencies it's a list of checksums for modules that were, at some point, used by this module. All those different versions of the same module listed there, they aren't all dependencies, at most one of them is.

Assuming 'go mod tidy' is periodically run go.mod should contain all dependencies (which in this case seems to be shy of 300, still a lot).

Half of go.sum dependencies generally are multiple versions of same package. 400 still a lot, but a huge project like gitea might need them I guess.

> cat go.sum |awk '{print $1}' | sort |uniq |wc -l

431

> wc -l go.sum

1156 go.sum