Submitted by gpersoon, also found by 0x0x0x, gzeon, harleythedog, hubble, and kenzo
When you cancel a promotion with cancelPromotion() then the promotion is complete deleted.
This means no-one can claim any rewards anymore, because  \_promotions\[\_promotionId] no longer exists.
It also means all the unclaimed tokens (of the previous epochs) will stay locked in the contract.
https://github.com/pooltogether/v4-periphery/blob/b520faea26bcf60371012f6cb246aa149abd3c7d/contracts/TwabRewards.sol#L119-L138
In the function cancelPromotion() lower the numberOfEpochs or set a state variable, to allow user to claim their rewards.
PierrickGT (PoolTogether) confirmed 
