This is the second time I've seen Lottie mentioned this week, without hearing about it before.
Maybe it's targeting a different use-case, but these things (at least on the Web) appear to be more-heavyweight and less-capable than the things people were doing 20 years ago with Macromedia/Adobe Flash, e.g. compare the animated-GIF-like examples linked from TFA ( https://thorvg-perf-test.vercel.app/ ) to the animations and games found on sites like Newgrounds. Last I checked, the latter make heavy use of emulators like Ruffle, or (based on loading screens) 3D game engines like Unity etc.
As someone who's been out of that scene for a long time: what's the overall state of things, if I want to make long, complex, 2D vector animations? (i.e. not using a 3D engine; and not rendering to video). SVG seems pretty established; but for animation, how capable is Lottie? Does anyone still use SMIL (outside of DVD menus)? Am I better off "rendering" to a big pile of JS + CSS transitions?
The problem is mostly that there doesn't exist some kind of standard for animators where they can make their animations in their editor and then export it for web.
Lottie started out as a plugin for Adobe After Effects to try and let them export animations for use on Web. As far as I know this is the only "half-standardised" way of exporting animations between tools.
If you don't need animations from a dedicated animator then the better solution is using "a pile of JS + CSS transitions", and hopefully this is what Lottie for the web eventually "compiles" into.
An alternative to Lottie that's seriously worth considering is Rive. This is personally what I'd choose for non-trivial use cases. https://rive.app
Sadly, the Rive app is basically online-only. I was psyched to see it that wasn't yet more Web-based stuff, but after installing the app found you have to set up an "account" and be online all the time to use it.
What a disappointing PITA.
Lottie is quickly becoming the de-facto standard for UI animations, but live, long running vector animations aren’t really something I’ve seen much of - at this point, video compresses well enough that people will simply use an mp4 or webp for that use case. I know that’s not what you’re looking for but since it isn’t a common use case, I haven’t seen much support for it. Lottie is perfectly capable for this use case, provided you don’t want audio.
I had heard of Lottie, but also saw the post you're referring to; and it called out Lottie for being bloated and hideously inefficient.