Like the title says after having installed kopia from source with makepkg -si I get a "go" directory in my ~/
do I need it or can I delete it?
Depends on the PKGBUILD, but usually during compilation, go downloads the dependencies into ~/go, but after compilation, the binaries are placed in /usr/bin and the dependencies are not needed, so you can remove it if you want. Bear in mind that in order to update it, you will need to run makepkg -si with the new PKGBUILD and the dependencies are redownload again. If you do not remove it however, it might save on some bandwidth.