Doesn't it depend on the topic/subject/level though? In a subject like Maths or science or MFL, then it's a fundamental requirement to have strong foundations before moving up to higher level/more complicated topics. For those "building block" subjects, I think it's essential for schools to start back where they left off.
For other subjects, such as humanities, English, etc., I think there's more scope to "forget" topics that were stand alone, one off, etc that later topics/modules etc don't depend on quite so fundamentally. (Obviously not in cases where required for GCSE/A level exams!). But, say, in a non exam year, if the class was going to do a project on, say, Ancient Egypt, or a particular piece of literature, or even in science/geography, a topic not inc in an exam such as rocks, etc., then it can simply be left out and forgotten for that cohort.
There is no "one size fits all". Dfiferent cohorts, different subjects, etc will have different needs. We really have to rely on teacher judgments as no one else can know what's best. The last thing we want is "top down" instructions from an incompetent quango similar to what Ofqual did with the grading!!