Yes — the small battery inside an earbud charging case is still lithium-ion and follows the same gradual capacity-loss pattern as any other rechargeable battery, which is why older earbud cases sometimes hold a full charge for noticeably fewer earbud recharges than when new.
Worth knowing
Small details like this rarely change a buying decision on their own, but they explain a lot of the charging behavior people assume is a problem when it’s actually normal.