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You might use set operations: complement and union.

Assuming that recent notifications limited to 100.

Set<Id> updateNotifications(Set<Id> persisted, Set<Id> recent) {
    Set<Id> newNotifications = complement(recent, persisted);

    if (recent.size() < 100) {
        return union(persisted, newNotifications);

    Set<Id> obsoleteNotifications = complement(persisted, recent);

    return newNotifications;