IGNORE { } instructions are working fine if a job does not exists. If they are nested in PARALLEL { } blocks, the build stops if an "ignore" job is failing.

For an example with a failing test case, see:

https://github.com/kremers/build-flow-plugin/commit/5aefdf131b35ea88e6ad6478d70e30dece34bd09

testIngoreInParallelBlock(com.cloudbees.plugins.flow.ParallelTest) Time elapsed: 10.406 sec <<< FAILURE!
junit.framework.AssertionFailedError: job job3 didn't run
at junit.framework.Assert.fail(Assert.java:57)
...