Function removed_units 

pub(crate) fn removed_units(
    base_m: &Manifest,
    cand_m: &Manifest,
    activation: Option<&ActivationPlan>,
    defs: &UnitDefs,
    services: &BTreeMap<String, Sensitivities>,
) -> (Vec<UnitId>, Vec<Sensitivities>, Sensitivities)

What a generation transition takes away (§11 Rollback row, §16): units present now and absent from the target, and units its activation stops, sorted, with their sensitivity. A removed user template’s definition is in the base manifest, not in defs (which holds the per-session instances), so it is looked up there: an unknown unit must not count as Routine (§11 sensitivity completeness, fail closed).