Is UI the only major change?

Saw the (apparently) excessive complaint about the new UI.

I haven’t seen it, but I wonder: if I were to dislike it enough to keep using my current version, would it remain compatible with AniWeb for synchronization? I prefer to use my iPad for study, but adding notes is much easier on the laptop.

Dae tries to keep things as backwards compatible as possible. Anki 2.0 was able to sync for a long time after Anki 2.1 arrived, but eventually support had to be dropped. The GUI changes are no big matter and are not a reason not to update. If you don’t need the newest and fanciest Anki, I’d rather recommend you sticking to the release cycles a few add-on developers have decided to adapt, such as lovac42 and Migaku (former MIA):

Development Cycle
This addon follows a six month development cycle to avoid problems introduced to addons due to the constant fluctuations made to the Anki API. I do not want to stop maintaining these personal projects, but it is extremely taxing on a developer’s schedule when we have other deadlines and obligations. Hopefully, adopting a six month development cycle following the MIA Team’s example could solve some of these issues.

Current Target and Cycle Details:
Anki Version: 2.1.22 (Windows and Linux), 2.1.23 (macOS)
Start Date: April 2020
Next Cycle Start Date: October 2020

So long story short: as long as you are on any 2.1 version and the add-ons you need are compatible, no real need to update. But also no real reason not to.