Conan, Spack, Nix, vcpkg, not to mention the OS specific ones. All are actively maintained. Modern means different things to different people, so I expect some people will still be looking for more answers.
- Conan and vcpkg are probably the closest equivalent to other "language package managers" (e.g. Rust's cargo, or NodeJS's NPM).
- Spack is typically used in HPC/Scientific Computing domain.
- Nix (and Guix) are very powerful (system) package managers; these have enough nifty features that I'd call them "next-generation".
Probably also for sufficiently complicated projects, it may be worth mentioning sophisticated build tools like Bazel (which I think manages dependencies in its own way).
There is windows support now (in addition to linux and macOS), which I think was a roadblock for many, and we have gotten some very interest from folks outside the HPC community, e.g. Replica.one, who hope to use it for a software-defined OS for embedded devices: https://www.youtube.com/watch?v=sMxNafpDhng (skip to ~12min for some discussion of Spack, Nix, Guix, and portage)