This shouldn’t even be necessary, as the older OS version is still there on the phone after an update, and a mechanism to revert back to it exists by design for A/B devices like the Fairphone 3/3+.
Lately, the question is whether the mechanism would work as intended … I’ve seen it not work for an /e/ user a while ago and in the meantime for a LineageOS user, too.
Fairphone OS? Who knows, either nobody dared to try or nobody dared to tell …