The article mentions they used an experimental language feature, which is enabled via a special compilation flag and came with a big warning from the start that it was in fact experimental and could be removed any time. I highly doubt that most people use any libraries that use those.

As a side note, I've personally had the experience of migrating several projects to Scala3 (when it was still called Dotty), and never had any issues with 3rd party dependencies, since Scala 3 is binary-backwards compatible with Scala 2.13

If they are in libraries it doesn't matter, because you can use Scala 2.13 code in Scala 3.

It definitely took a while for most libraries to catch-up with Scala 3, now, I don't see anything is missing in the main area I work with (web development), playframework already works for Scala 3.