I had a similar experience, i.e. strict keto for 9 months and fully came off meds with no signs of UC or asthma. UC came back after around 2 years and I don't remember how long before I had to start asthma meds again. I can no longer do keto though, each time I try I get palpitations now after around a week.
Yeah my asthma went into remission as well, though this was also a period of my life when I was very physically active and in the best shape of my life, so it's hard to say what was the cause there.
They sometimes can, but most properly gut-adapted microbes only have their relative prevalence reduced by diet changes, by and large people unfortunately tend to snap back once diet is discontinued.
Ideally, you'd combine that with doing a stool transplant first, since it's the main thing (other than just antibiotics) that causes permanent compositional changes.
In my n=1 experience, it seems maybe significant dietary changes can perturb the gut ecosystem out of whatever state corresponds to IBD.