go: sum.golang.org: go-get gitern.com/xxx/repo.git 410 gone

What version of Go are you using (go version)?

$ go version
go version go1.14.6 linux/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/user/.cache/go-build"
GOENV="/home/user/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/user/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/tmp/trash/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build897724545=/tmp/go-build -gno-record-gcc-switches"

What did you do?

go get gitern.com/urandom/errors.git

What did you expect to see?

go: downloading gitern.com/urandom/errors.git v0.0.0-20200723020514-49681dcbcc21

What did you see instead?

go: downloading gitern.com/urandom/errors.git v0.0.0-20200723020514-49681dcbcc21
go get gitern.com/urandom/errors.git@master: gitern.com/urandom/errors.git@v0.0.0-20200723020514-49681dcbcc21: verifying module: gitern.com/urandom/errors.git@v0.0.0-20200723020514-49681dcbcc21: reading https://sum.golang.org/lookup/gitern.com/urandom/errors.git@v0.0.0-20200723020514-49681dcbcc21: 410 Gone
        server response:
        not found: gitern.com/urandom/errors.git@v0.0.0-20200723020514-49681dcbcc21: invalid version: git fetch -f origin refs/heads/*:refs/heads/* refs/tags/*:refs/tags/* in /tmp/gopath/pkg/mod/cache/vcs/c58e8953608705577f340d5d716eb03f031f4f09a3698dde27ce667a4baf661a: exit status 128:
                fatal: unable to connect to gitern.com:
                gitern.com[0: 100.20.209.4]: errno=Connection refused

The repo appears correctly populated in ~/go/pkg/mod/cache, implying that proxy.golang.org is working, plus hitting vcs directly, via GOPRIVATE=gitern.com, works as expected.