Using the following in my declarative pipeline:

environment {
  PATH = '/usr/bin:$PATH'
}

Gives this warning at build time:

Warning: JENKINS-41339 probably bogus PATH=/usr/bin:$PATH; perhaps you meant to use 'PATH+EXTRA=/something/bin'?

The problem is, the "PATH+EXTRA" syntax this warning is suggesting doesn't seem to be available in declarative pipelines. Also, the issue mentioned in this warning seems to be about scripted pipelines only.