If I got warts on the bottom of my foot once already (and got treated), does that mean I am immune to them going forward? Or still a risk if exposed?

Wart. You can still get warts. Getting a wart once and getting it treated does not mean that you are immune to it. Wart is caused by a virus.
Still at risk. Re -occurence is common.
Yes. Our understanding of immune system is still an art and inexact science. A person exposed to a particular infection such as chicken pox will likely not develop chicken pox but still develop shingles. In aids, we have seen recurrence of diseases due to suppressed immune system. Similarly your immune response to wart can diminish over time and you can be reinfected.