It's not a myth at all, off the top of my head I know of at least two cases studies including the study that led to the discovery that remdesivir may treat polio

https://pmc.ncbi.nlm.nih.gov/articles/PMC10022663/

Immunocompromised hosts can shed infectious polio for their entire lives.

These outbreaks sometimes die on their own, but there's no reason to believe they wouldn't spread widely if we completely stopped vaccinating