> It's not toxic, and simple to test on yourself, and you've probably already got a bottle of lemon juice in the refrigerator.

Lemon juice contains a vast array of different chemicals, some of which may be phototoxic. It also releases VOCs (e.g. terpenes). I don't know how much compared to your average deodorant, though.

I don’t think phototoxicity is much of a concern for something meant to be applied to one’s armpits...

I would think the sugar in lemon juice might be a little sticky (presumably the bacteria in that environment aren't used to having fructose)? Likely to decrease the pH you could use vinegar, among other things.