Several places in the documentation for pipeline have examples that use _ as a throw away identifier.

Java (yes I know this is groovy) started warning that the use of underscore as an identifier would be removed in a future versions of the language and be made a reserved word.

as groovy and the JVM / java lang are somewhat tied it would not suprise me if this in the future started to fail in Groovy.

Therefore I recommend to not use _ in the documentation at all but use ignored or theLib as most people will copy it and then at some point with a JVM upgrade all the pipelines may break.