Okay so I had a few minutes to look into it today; turns out this is because the plugin is deployed at v0.12 but a valid semantic version tag is v0.12.0. So the plugin author will need to use semantic versioning for tags going forward.
I say this, because the error logged is:
2019/05/26 00:34:59 exec /usr/local/bin/go get -d -x github.com/echocat/caddy-filter@v0.12
mkdir -p /gopath/pkg/mod/cache/vcs # git2 https://github.com/echocat/caddy-filter
# lock /gopath/pkg/mod/cache/vcs/5c53692df796a3782f871f452d4f0c4345bc30e6e6535ff60b4a9b8b94c3d151.lock# /gopath/pkg/mod/cache/vcs/5c53692df796a3782f871f452d4f0c4345bc30e6e6535ff60b4a9b8b94c3d151 for git2 https://github.com/echocat/caddy-filter
cd /gopath/pkg/mod/cache/vcs/5c53692df796a3782f871f452d4f0c4345bc30e6e6535ff60b4a9b8b94c3d151; git ls-remote -q https://github.com/echocat/caddy-filter
0.269s # cd /gopath/pkg/mod/cache/vcs/5c53692df796a3782f871f452d4f0c4345bc30e6e6535ff60b4a9b8b94c3d151; git ls-remote -q https://github.com/echocat/caddy-filter
go get github.com/echocat/caddy-filter@v0.12: no matching versions for query "v0.12"