[GabrielTFS]

I would guess a significant portion of people using the style (if not most), did so inspired by Windows, though

[pjmlp]

It was common across all not UNIX operating systems.

You will find it on MS-DOS, Amiga, OS/2, Mac OS.

It was based on what was common in ALGOL derived languages.

On UNIX you will find it on X Windows, and Motif.

[uxp100]

Macintosh toolbox was Pascal first right? Or at least there was an era where it was. And I think this naming convention is kinda a pascal thing.

[pjmlp]

Yes, Apple is the actual creator of Object Pascal variant, and there was a Pascal based OS using P-Code for Apple II GS.