Riot's Design Director on Legends of Runeterra, Andrew Yip, has confirmed that we'll be seeing set rotations "inevitably".
More interesting than the confirmation of set rotations though is that they aren't sure about how they'll support the "all cards" format where you can play with the entire pool. Now, why is this interesting? Last week we saw the confirmation that we won't see card refunds when they make balance changes because of the philosophy that they'll quickly change cards that they make unviable to do changes, ensuring your time acquiring cards isn't being thrown out the window because of a patch.
One may be considered crazy to assume cards they earned years ago were going to be held to the same standard as newer ones, and this is especially true with a rotating format. Although details on how they'll handle a rotating format aren't yet available, it looks like there's a good chance once a card isn't in the "main" pool of cards for play, it may not be considered for updates and balance.
We've got a long way to go with Runeterra before we're going to need card set rotations though, so for now, get out there and enjoy the game! Make sure you're making good progress on your vault to maximize your rewards weekly since we don't need to be afraid of not being able to use our wild cards and shards for future set releases - they aren't going anywhere!
Are sets going to be rotated out by seasons similar to hearthstone or MTG? Or is runeterra taking a more yugioh approach where your card pool will be always be around to use in the main format.
Inevitably we will rotate - I think the critical question is how much we support the “all cards” format vs the more constrained one. That’s tbd for now!