When I’m not feeling lazy, I use the field description as a place to document formulas, since Airtable currently does not support comments in code. Since for the foreseeable future we’ll continue to rely upon such out-of-band documentation, this seems the obvious place to track formula creation and modification, as well. (In fact, it might not be that bad a place to surface the values if and when they are generated automatically.)
Note that any enhancements to formula field functionality should also be applied to rollup fields, given the currently undocumented but reportedly canonical support for aggregation formulas entered in the aggregation function configuration window.
