It's useful to distinguish between command line and GUI apps vs. the operating system itself.
Forward slashes and backslashes have been supported as path delimiters all the way back to MS-DOS 2.0 when hierarchical directories were first introduced. The DOS and Windows file APIs never cared which you used.
It was only command line and GUI apps that (rightly or wrongly) preferred one or the other. The only new thing here is that some of these apps have finally started to recognize either delimiter.
That is a tricky thing for a command line app where the wrong slash may be interpreted in a way the user did not mean. For GUI programs there was never a good reason to not accept both, e.g. in a file path field.
