Yes, I do.
I use a diary. If I used a separate app to remember vet appointments, I'd also need separate ones for my hairdressing appointments, my doctor or dentist appointments, and my meet-ups with friends. One diary (on my phone, as it happens) keeps all my appointments in one place and makes sure there's no overlap or double booking.
These are too important to forget but if I needed something, I'd simply use the alarm function on my phone.
I really don't see how an app could analyse something so complex with any accuracy. For example, a common problem is reactivity. But how does an app take account of trigger stacking, hormonal factors, sleep patterns, pain, weather etc that might contribute? My dog isn't very reactive but some other dogs make him tense - that's due to their body language, and I can read that and make sure my dog has enough distance to feel comfortable. An app isn't going to do that, so suggesting it might help with behaviour is naive, and even potentially dangerous because it oversimplifies things.