os-agnostic is a classic example of a meta-tag. It doesn't provide any meaningful information and should be deleted.
I see this one as an OS tag replacement rather than a meta tag. It's for questions in which the specific OS often makes a difference but for that particular issue, it doesn't, and it conveys that fact.
It could be a situation where a problem is known to be across all platforms on which an application runs, so it is relevant to include this as diagnostic information. Another situation is where someone has access to every major OS, is looking for a solution to a problem, and isn't particular about the platform. I answered a question like that, which got closed, and one of the stated issues (in comments), was that a specific OS wasn't specified.
The tag replaces inclusion of every possible OS tag. It could be renamed any-os. However, it should get a wiki excerpt clarifying that it should be used only for questions that are exceptions to the usual relevance of a specific OS. I imagine that lacking such a definition, past usage of the tag wandered from a legitimate purpose.