It'll just degrade gracefully into not animating the element's entry, so unless the animation is somehow crucial you should still be fine to use it.
If you really need to detect whether it's supported there are hacky methods: https://www.bram.us/2024/07/11/feature-detect-css-starting-s...
It depends on what you're doing. It's common for clients to wonder why the design they saw had fancy animations yet they don't see them on their MacBook...
I agree, but must also observe that I have never met a designer who was willing to admit without a knock-down drag-out fight that any animation they put in was not somehow crucial.