No — your miles in Atmos Rewards (formerly Mileage Plan with Alaska Airlines) do not expire simply due to passage of time.
However, your account may become inactive if you haven’t earned or redeemed any points for more than 24 months; at that point your account can be locked and if it remains locked for a further period it could be closed — which would result in forfeiting your points.
However, your account may become inactive if you haven’t earned or redeemed any points for more than 24 months; at that point your account can be locked and if it remains locked for a further period it could be closed — which would result in forfeiting your points.