Virus mutates to a less severe form, quarantine measures select for the less severe form, fighting off less severe form provides immunity against more severe form, severe form dies out.
Another theory holds that the 1918 virus mutated extremely rapidly to a less lethal strain. This is a common occurrence with influenza viruses: There is a tendency for pathogenic viruses to become less lethal with time, as the hosts of more dangerous strains tend to die out[15] (see also “Deadly Second Wave”, above).
Virus mutates to a less severe form, quarantine measures select for the less severe form, fighting off less severe form provides immunity against more severe form, severe form dies out.
According to https://en.wikipedia.org/wiki/Spanish_flu
Article today suggested that COV19 has already split into two strains and hypothesized that selection pressure from quarantine changed the relative frequencies of the strains, don’t think there’s evidence about whether one strain is more severe https://academic.oup.com/nsr/advance-article/doi/10.1093/nsr/nwaa036/5775463?searchresult=1
I’m not an expert and this isn’t great evidence, so it’s maybe in the “improbable” category