The description of the Haste spell concludes with:
When the spell ends, the target can't move or take actions until after its next turn, as a wave of lethargy sweeps over it.
Could the target still use a reaction (to, say, throw up a Shield spell if needed) or is a reaction considered a type of action?
Since the spell depends on concentration, it could end unexpectedly if the caster loses concentration on it, leaving the target in a perilous situation.
Best Answer
Yes, you can
Jeremy Crawford confirms this on Twitter.
Other supporting evidence
In the PHB, the Incapacitated condition differentiates between actions and reactions:
Ramifications
Actions and reactions are different.
If you had haste and fly on (concentration can be dealt with via potion, buff from ally, magic item, etc), and someone cast a dispel magic on you, you can still catch yourself with a feather fall.
If you were frozen with lethargy, you can still muster the "energy" to cast shield, hellish rebuke, absorb elements, or even counterspell, and perhaps you can narrate it as one desperate wave of the hand and a breathless incantation.
Ending haste does not incapacitate you. It just makes you lethargic and unable to take actions.