Would the Slow spell block a Hunter ranger from using Volley or Whirlwind Attack?

Question

The Slow spell includes this rule for any creature affected by it:

Regardless of the creature's abilities or magic items, it can't make more than one melee or ranged attack during its turn.

A Ranger with the Hunter archetype gets the Multiattack feature at level 7, letting them choose to gain either Volley or Whirlwind Attack:

Volley

You can use your action to make a ranged attack against any number of creatures within 10 feet of a point you can see within your weapon’s range. You must have ammunition for each target, as normal, and you make a separate attack roll for each target.

Whirlwind Attack

You can use your action to make a melee attack against any number of creatures within 5 feet of you, with a separate attack roll for each target.

As I understand it, both of these abilities are considered to be a single attack, albeit with multiple attack rolls. Does that mean the Slow spell would not prevent a ranger from using either ability if they were under its influence?

Answer 1

Volley and Whirlwind Attack are (according to the Sage Advice Compendium) a single attack, so the slow spell has no effect on them

We can see the following from the Sage Advice Compendium (page 4):

Q. Can a ranger move between the attack rolls of the Whirlwind Attack feature?

A. No. Whirlwind Attack is unusual, in that it’s a single attack with multiple attack rolls. In most other instances, an attack has one attack roll. The rule on moving between attacks (PH, 190) lets you move between weapon attacks, not between the attack rolls of an exceptional feature like Whirlwind Attack.

The following question establishes the case for Volley also being a single attack (its wording matches that of Whirlwind Attack's): "Is the Hunter ranger's Volley feature considered a single attack or multiple attacks?"

As such we can now consult the slow spell:

[...] Regardless of the creature's abilities or magic items, it can't make more than one melee or ranged attack during its turn [...]

However, as established above, Whirlwind Attack and Volley are both only a single attack, and thus the slow spell cannot limit them in any way.

Answer 2

The wording on the abilities is a bit confusing, and you can't easily discern the real meaning of the wording in this case. Luckily, the Sage Advice Compendium contains the following official ruling:

Can a ranger move between the attack rolls of the Whirlwind Attack feature?

No. Whirlwind Attack is unusual, in that it’s a single attack with multiple attack rolls. In most other instances, an attack has one attack roll. The rule on moving between attacks (PH, 190) lets you move between weapon attacks, not between the attack rolls of an exceptional feature like Whirlwind Attack.

According to this, the Whirlwind Attack feature counts as one attack that involves multiple attack rolls. By the same logic, Volley would have the same detail applied to it as well, and so neither would be limited by the slow spell.