I recently came into possession of a large maple butcher-block style countertop, which previously was used as a desktop. I don't know the full history of the wood, but it has some sort of finish on it. I would like to plane off the finish / glue and then repurpose the wood for a cutting board. Is this safe to do? In general, how far into the wood do finish volatiles penetrate?
If I remove old finish from wood, is the wood food-safe?
Once any solvent residue1 has evaporated, yes.
If done well, and especially using an effective stripper, stripping can remove all finish from wood as far as we can tell. Any planing or significant sanding will only do more to remove possible traces of finish from areas of greater absorbency.
However, if the surface of your maple countertop material is in good shape so you're only intending to do rudimentary sanding any remaining traces are absolutely nothing to worry about because the weight of evidence is that all wood finishes are food safe2.
So regardless of how thorough you are the wood will be safe for direct food contact.
In general, how far into the wood do finish volatiles penetrate?
If we take out the word volatile here and just consider finish penetration, surprisingly shallowly. Certainly less deeply than finish companies often 'suggest' in their product information, and even more so in their marketing blub ^_^
The word volatiles is key here though, there are none left in dried finish. This is precisely because they're volatile — they evaporate, fully, from any finish that contains them. And in quite a short time. Once any solvent-containing finish is cured enough to use they are already long gone.
1 If any; not all strippers contain solvents.
2 The whole "food safe" thing is entirely a red herring.