This is discovered while I'm review another PR. And there are already some tests that using checkUnchanged, but the xml file still contains planAfter section, such as RelOptRulesTest#testNoReduceAverage.
IMO, there is no need to keep planAfter for checkUnchanged, and it does more harm than good to keep it. Even we can make planAfter the same with planBefore, it will bring burden for reading (because we'll identify the difference of planAfter and planBefore). And it will be more harmful when planAfter is different from planBefore.