To be fair, armpits are pretty quick to shave and I could see some people sticking with some personal hygiene habits in times like that just to feel a bit more normal. Also, if they don't have time for showering, shaving armpits might be one of the most effective ways to keep the smells lower, like the difference between needing to scrub a lot with a sponge or just give it a wipe.

Personally, I shave my face because once it gets to a certain length, it annoys me. Same with my head. Now I do use an electric razor for both of those and don't go down to the skin for either, so whether I kept up with that or went all mountain man would depend on access to electricity, but if I had that access, I would absolutely spend a bit of time managing that because it starts to be a distraction if I don't.