The Stone Golem has an ability called Slow:
Slow (Recharge 5-6): The golem Targets one or more creatures it can see within 10 ft. of it. Each target must make a DC 17 Wisdom saving throw against this magic. On a failed save, a target can't use reactions, its speed is halved, and it can't make more than one Attack on its turn. In addition, the target can take either an action or a Bonus Action on its turn, not both. These Effects last for 1 minute. A target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
The Slow spell has a similar effect.
My immediate thought was that this is aimed at weapon attacks eg. a fighter's extra attack or a monk's flurry of blows. Then I wondered, what about spells that have an attack roll eg. Eldritch Blast. At higher class levels that spell allows multiple attack rolls. Does the slow effect also prevent you from making the extra attacks with that spell?
What about class abilities like Volley or Whirlwind Attack from the Ranger - Hunter subclass?