In the go.mod, there are two dependencies github.com/sagernet/sing and github.com/sagernet/sing-shadowsocks that are under GPL-v3 or later, since the remaining code and these two dependencies in the project are static links, so the entire project should use GPL-V3 or Later(actually the combination of MPL-2.0 and GPL-v3 or later).

PS: According to the GNU officialhttps://www.gnu.org/licenses/gpl-faq.en.html#GPLStaticVsDynamic, no matter how the GPL code is being used (statically or dynamically linked), the terms and conditions of the GNU General Public License cover the whole combination.

There is an exception https://www.gnu.org/licenses/gpl-faq.en.html#LinkingOverControlledInterface that if the copyright holders of the project under GPL can authorize the linking exception with the condition of not modify the GPL project's code. However, it seems that the auther(s) of the github.com/sagernet/sing and github.com/sagernet/sing-shadowsocks doesn't claimed that. Maybe negotiation with the copyright holder(s) of the github.com/sagernet/sing and github.com/sagernet/sing-shadowsocks is needed.

Of course, I'm not the layer and my opinion might not be so accurate.