The Filesystem Hierarchy Standard says where to put stuff in a UNIX distribution.
Is the FHS used/designed for use outside of GNU/Linux, or is it mostly limited to GNU/Linux?
According to the Wikipedia page the standard is for "Unix and Unix-like operating systems". While it may have grown out of a predominantly GNU/Linux environment, the intention seems to have consistently positioned it as focused on the broader *nix world.
The first version, originally bearing the catchy name, FSSTND, was published in 1994. The accompanying FAQ describes how it originated:
The FSSTND is a consensus effort of many Linux activists; the main arm of their discussion takes place on the FSSTND mailing list... The FSSTND draws ideas from POSIX, 4.4BSD, SVR4, SunOS 4, MCC, Slackware, SLS, (in no particular order) and many other systems. We have not followed any one operating system layout in entirety. Instead we have tried to take the best of each filesystem layout and combine them into a homogenous whole, well suited to the needs of Linux users everywhere.1
The Linux Foundation is currently working on the next version, FHS 3.0, and clearly indicated they see it as applying to the wider Unix ecosystem:
The Filesystem Hierarchy Standard (FHS) is a reference describing the conventions used for the layout of a UNIX system. It has been made popular by its use in Linux distributions, but it is used by other UNIX variants as well.2
As to whether in practice the FHS is widely adopted: it is, but inconsistently.
I'd read the FHS wikipedia page. It pretty much sums it up if I understand your question correctly.
The Filesystem Hierarchy Standard (FHS) defines the directory structure and directory contents in Unix and Unix-like operating systems. For the most part, it is a formalization and extension of the traditional BSD filesystem hierarchy.
Based on this I would interpret it as specifically designed for Unix distributions, not necessarily just GNU/Linux.
Despite the efforts to widen its audience, the FHS is mostly relevant only to Linux distributions and ignored by BSD and commercial Unix vendors (see this posting)
Its third release is still a draft after many years, and it looks like nobody is actually working on it these days.
Trying to evolve or refine a standard about things that are quite established and sometimes contradictory between distributions, not to mention different operating systems, is likely a frustrating and tedious job.
Can be used on any POSIX system. BSD, Linux and Unix all basically follow it.