Originally Posted by EvilAngel

Usually, no. Just the current alpha's as beta's in their next coliseum.

So far no cards have been removed from SR-Gacha so I am prity cetain that the cards in SR-gacha stay there.

When an event starts all old slayer cards are removed from E-gacha and the cards that become slayer card in next event are added.
I only spend my e-gacha in 2nd half of en events because chance of getting slayer cards are 50% higher in 2nd half.
Normalt a free player like me can get around 2 e-gacha per event. So normaly I end up with one R-Slayer per event.

Most old slayer cards reapear in one future event. I dont think any slayer card have been in more than 2 event.