Earlier quoted context omitted.
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...
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.
The designers I've worked with are fine with these things. We have more important things to work on than small style details. We can go back and change these things later if anyone actually cares, but generally nobody ever does.