Akali, Silent (Alternate Art)

Yes, Akali can be targeted by enemy [Deathknell] abilities until combat ends, as long as she remains at the battlefield with a combat designation.

Combat has three steps:

1. Combat Showdown Step.⁴⁶⁶

2. Combat Damage Step.⁴⁶⁷

3. Resolution Step.⁴⁶⁸ The combat cleanup kills lethally damaged units.^468.1323.4 A [Deathknell] trigger is added to the chain before its unit moves to the trash.^808.1.d.2 Any targets are chosen when those abilities finalize.^403.2 Those abilities fully resolve before combat proceeds to determining the result.^468.2 Combat ends only at the final part of this step.^468.7 Only then is Akali's combat designation removed, so she stops being in combat and her restriction against enemy spells and abilities applies again.^{468.7.a740.2.c}

This is why Ruined Rex's [Deathknell] can choose her after Rex dies to combat damage.