"Immunity debt" is horseshit from medical POV, there is no acquired "immunity" at all to RSV or Corona viruses (like the common cold), not in the way we think of vaccine-conferred immunity for rubella or mumps etc. So already "immunity" part of "immunity debt" is clearly a misnomer.
It is a train thought that suggests exposure to pathogens is good for you - a dubious claim, on it's face - and fundamentally misrepresents immune function.
" In fact, Furness likens the immune system to a collection of
photographs. When people take photos and put them away in an album, the
photos don’t fade over time just because they aren’t being looked at
regularly."
https://globalnews.ca/news/9272293/immunity-debt-covid-19-misinformation/
Plenty more -
https://www.ft.com/content/0640004d-cc15-481e-90ce-572328305798
An alternate explanation for severe RSV outcomes among children, no dubious claims anent immune system necessary - their lungs are damaged from COVID infection and they're more susceptible to hospitalization from RSV as a result. And there you have the distractive, CYA utility of pushing "immunity debt" bologna.
https://medicalxpress.com/news/2022-09-lung-children-teens-covid.html