Indeed, at this point it's one of the most devalued terms in software. Every week I'm reading about some product calling itself "the Operating System for XYZ." It seems like it's what you do when "platform" just feels too modest. To me, if it's not bootable, calling your product an OS just reeks of marketing sleight of hand.

Because "The Android of ..." is out of fashion?

In some sense, it is. Sandstorm called itself an OS too. Arcan calls itself an Overlay-OS.

I think for any platform for general purpose computing that provides an application model that programs are written to target, it's close enough I don't mind. Ie: it makes sense for some end-user software to have a target for Windows, Linux, Sandstorm, etc.

& from the other side- what does Debian add that lets it call itself an OS? A set environment and package manager that software is specifically packaged for.