In the context of card sharing, a booster card is a card that is not introduced during the original lesson, but is rather introduced much later.
I think that something like Quantum Country would benefit from having additional cards that you can collect by reading a separate page a month or so after you read the original article. It would cover cards that are less essential but still solidify your understanding -- the stuff in the original article that would have "fallen through the cracks" if you just did the given prompts without re-reading the article. These additional facts might be annoying to collect on a first pass, as you are still trying to make sense of the "main thread" of the article, but become important over time. This might be one escape from Feeling like a perpetual student in a subject due to spaced repetition, and Andy Matuschak's Efficient review scheduling is in tension with gated course sequences.
Booster cards can also work nicely when a piece of knowledge has multiple prerequisites. For example with Tao Analysis Flashcards, when the successor operation ++ is introduced in Section 2.1, the reader does not yet have the vocabulary to state that ++ is an injective function (even though this is fact is already implicit in the Peano axioms). Once functions and injectivity are introduced in Chapter 3, Tao does not go out of his way to highlight that ++ is an injective function. But a booster card could be added that reinforces the old knowledge given this new vocabulary.