#28680 is fixed and go clean --modcache command no longer donwloads modules before cleaning if no argument is given to the command.

Note however 'go clean' accepts packages parameter as an argument(which isn't relevant to -modcache but is still meaningful for other purposes of go clean). If they are given, go clean still downloads the modules and even updates the go.mod file.

$ go help clean
usage: go clean [clean flags] [build flags] [packages]
...
$ export GOPATH=$(mktemp -d)
$ go1.13beta1 clean -testcache -cache -modcache
$ go1.13beta1 mod init scratch
go: creating new go.mod: module scratch
$ go1.13beta1 clean  golang.org/x/text
go: finding golang.org/x/text v0.3.2
go: downloading golang.org/x/text v0.3.2
go: extracting golang.org/x/text v0.3.2
$ cat go.mod
module scratch

go 1.13

require golang.org/x/text v0.3.2 // indirect

And the go clean -modcache <package> behaves differently.

$ go1.13beta1 clean -modcache golang.org/x/text
$ go1.13beta1 clean -modcache golang.org/x/text
go: downloading golang.org/x/text v0.3.2
go: extracting golang.org/x/text v0.3.2
go: finding golang.org/x/text v0.3.2