On a Macbook, It's good to use "fn" key as sprint key. It's easy to use than the Ctrl key.
I use it all the time in old versions of Minecraft on my Mac. But it doesn't work now. Please bring it back.
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Yes, in 1.12.2. and all past versions can use "Fn" key. Not like a PC keyboard, ctrl on the left corner . Fn key is on the bottom left corner on a Mac keyboard, the ctrl key is on the right of it. So it's more easy to use it as sprint key.
The engine of snapshot version looks changed a lot, I don't know why the new one can't access this key. But I think this function is really convenient to our mac user.

MC-29051 probably
Usually the Fn key is only to access other keys on your keyboard, so it's actually not a real key. At least it's like that on Windows, I have no clue how it works on macOS.
Also, just to make sure: You were use the Fn key as a control setting in 1.12.2, right? Is that what you mean by "old versions"?