[agapon]

Underscore is (was?) illegal in domain names.

[daenney]

It's not illegal in domain names. Take a look at the SRV type in the domain name system. It's underscores galore.

However, for host names or for a URL's host component, you're limited to hyphens.

In this case though it's just the name of a Julia package[1], which isn't bound by either the rules of domain names or host names as .jl is not a TLD.

[1]: https://pkgdocs.julialang.org/v1/creating-packages/#Package-...