post-scala/scala#6436, scala-xml is no longer bundled with Scala and no longer available by default in the Scala REPL:
Welcome to Scala 2.13.0-pre-91a7d72 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_172).
Type in expressions for evaluation. Or try :help.
scala> <foo/<>
^
error: in XML literal: '>' expected instead of '<'
scala> <foo/>
^
error: To compile XML syntax, the scala.xml package must be on the classpath.
Please see https://github.com/scala/scala-xml for details.
that probably means an influx of visitors to this repo. we should either make sure the README.md here helps them, or put information up at a better URL and change the error message.