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I did exactly that with a VBA macro, that indicated whether something was a formula, a constant, or empty. I would just put it next to my column of calculations. It was not what I would call "clean code" but sure made it easier to follow my thought process and find bugs.

The macro was the first thing that came to mind. Your formula is cleaner.

You could make it a matter of style discipline. In my non-trivial spreadsheets I typically have cell formatting conventions, e.g. light yellow background is input, light green is formula, etc. You do have do it manually but it's worth the effort in my case.

And that shortcut is very annoying to type on a German keyboard layout.

Why not ISFORMULA