They used to work until the big modules change a few golang releases ago. Now, nothing works anymore unless you exhume a sufficiently old golang compiler.
I see what you mean about this particular project, but more generally, you can certainly use pre-modules Go code with current Go compilers. See e.g. https://github.com/golang/go/issues/37797 (a case of someone not RTFMing, but the response illustrates how it's done).
If you just want to pretend modules never happened, then export GO111MODULE=off and use the latest Go compiler.
In most cases, these things would work, but not here. Tried everything, nothing works except using a pre-modules-compiler.
In general, golang seems to work best with golang-only projects. As soon as there are other languages in the mix, like C++ here, things get very ugly very fast.
Ah, I think I misinterpreted what you meant by "nothing works anymore". I thought you were saying that no old Go projects were usable with new Go compilers. I believe you that this particular project may not work with the current toolchain.
Ah, yes, that wasn't sufficiently clear in my text. In almost all projects I've done/used, go modules work fine and are nice. It is just this one library that I know ceased to work.
The project also seems dead since 2020 or so, a current fork is https://github.com/bluszcz/cutego/ But I haven't tried that one.