When did the colon character : become an allowed character in the filesystem?

Just noticed it works on Maverick.. the colon character : is valid for file and directory names (I accidentally entered it for a filename in Atom).

Can't find any reference to it becoming valid though (http://ift.tt/1cuveq1 still marks it as invalid).

It works in the Atom editor.

I can create a file called : with nano.

This works echo 'test' > : && cat :

In TextWrangler it only works if the file already exists but won't create it (with command line tools installed edit :)

Is this new from Mavericks? Works in Yosemite? Before?

Edit: A few more tests:

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  TextEdit won't allow me to enter it and always replace it with -

  
  


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  Finder won't allow me to create a folder with the name :

  
  


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  A : file created via command line is displayed as / in Finder.. and double clicking it will open it in TextEdit. TextEdit will load its content but show the name / in the title bar.