Radishes deter flea beetles.
Contested
Radishes don't repel flea beetles - flea beetles like radish, which is why some gardeners use it as a trap crop and sacrifice it. If your radish leaves are riddled with tiny holes, that's flea beetles doing exactly what they do; it isn't protecting your other brassicas by scent.
What is actually known
Radish does not repel flea beetles; if anything it draws them. Flea beetles feed readily on radish foliage, which is exactly why radish is sometimes planted as a deliberate TRAP crop - the beetles concentrate on the sacrificial radish and you accept its holed leaves to spare the main crop. That is the opposite of a deterrent, and it works only if you are willing to lose the radish and pull it before the beetles move on. The same claim is made for radish "protecting" cucumbers or squash, with the same correction.
Evidence: not yet confirmed against any source.
When would that grow at your place? →
Cite this page
Milpa Gardens. "Radishes deter flea beetles." Evidence grade: Contested. https://milpa.garden/beliefs/radish-flea-beetle/
A garden-planning corpus: every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.