It's not "immunity debt" but rather that immunity for seasonal diseases is not permanent - you lose it if you don't use it, basically, with "refresh period" about the same length as the disease cycle.

I don't have citations handy, it's just something I've seen mentioned here and there some months ago, as a possible explanation why after 2 years of lockdowns, many more people seem to catch the common seasonal diseases.

It seems plausible that the immune system requires stressors to function well, and the absence of stressors may reduce immune system "strength". Selye's general adaptation syndrome seems applicable here as a first approximation. Muscles atrophy from disuse. Why not also immune systems?