Not even on Windows?

No, file I/O on Windows in general doesn’t use UTF-16, but the regional code page, or nowadays UTF-8 if the application decides so.

Depends on what you define as "file I/O", though. NTFS filenames are UTF-16 (or rather UCS2). As far as file contents, there isn't really a standard, but FWIW for a long time most Windows apps - Notepad being the canonical example when asked to save anything as "Unicode" would save it as UTF-16.

I'm talking about the default behavior of Microsoft's C runtime (MSVCRT.DLL) that everyone is/was using.

UTF-16 text files are rather rare, as is using Notepad's UTF-16 options. The only semi-common use I know of is *.reg files saved from regedit. One issue with UTF-16 is that it has two different serializations (BE and LE), and hence generally requires a BOM to disambiguate.

Powershell use to output utf-16 by default on Windows. It might still but it's been awhile since I needed to try.

Then you're talking about the C stdlib, which, yeah, is meant to use the locale-specific encoding on any platform, so it's not really a Windows thing specifically. But even then someone could use the CRT but call wfopen() rather than fopen() etc - this was actually not uncommon for Windows software precisely because it let you handle Unicode without having to work with Win32 API directly.

Microsoft's implementation of fopen() also supports "ccs=..." to open Unicode text files in Unicode, and interestingly "ccs=UNICODE" will get you UTF-16LE, not UTF-8 (but you can do "ccs=UTF-8"). .NET also has this weird naming quirk where Encoding.Unicode is UTF-16, although there at least UTF-8 is the default for all text I/O classes like StreamReader if you don't specify the encoding. Still, many people didn't know better, and so some early .NET software would use UTF-16 for text I/O for no reason other than its developers believing that Encoding.Unicode is obviously what they are supposed to be using to "support Unicode", and so explicitly passing it everywhere.

Or possibly confusing it with JavaScript, which treats strings as sequences of UTF-16 characters?