I do care, and in your example, it depends on what license the project uses and how "proprietary_library" is distributed.
In this specific case, if Muon distributed a copy of Ultralight (which it doesn't seem to; I'm not sure why I'm spending so much time on this), the it could not be GPL'ed, for example, because Ultralight has a proprietary (incompatible) license [2]. For a license like MIT or BSD, I think applying that license is technically valid, but again, not very practical. I doubt Muon would make it into the OpenBSD repos, for example. Its distribution is hindered by the depedency.
Basically, "open source" doesn't really mean anything in this context; you need to consider specific licenses and circumstances.
In this specific case, if Muon distributed a copy of Ultralight (which it doesn't seem to; I'm not sure why I'm spending so much time on this), the it could not be GPL'ed, for example, because Ultralight has a proprietary (incompatible) license [2]. For a license like MIT or BSD, I think applying that license is technically valid, but again, not very practical. I doubt Muon would make it into the OpenBSD repos, for example. Its distribution is hindered by the depedency.
Basically, "open source" doesn't really mean anything in this context; you need to consider specific licenses and circumstances.
[1] https://github.com/ImVexed/muon/tree/master/ultralight
[2] https://github.com/ultralight-ux/Ultralight/blob/master/lice...