The slow spell states:
[...] Regardless of the creature's abilities or magic items, it can't make more than one melee or ranged attack during its turn [...]
I realized that one way around this restriction is to use attacks that target multiple enemies. For example the Hunter Ranger's Whirlwind Strike and Volley features. The answer to the linked question explains Volley which Jeremy Crawford, lead game designer, has tweeted matches the intended reading:
Like Whirlwind Attack, Volley is a single attack with multiple attack rolls.
Regarding Whirlwind Attack, the Sage Advice Compendium explicitly states:
[...] Whirlwind Attack is unusual, in that it’s a single attack with multiple attack rolls [...]
Another feature I've found that does this is the Battle Master Fighter's Sweeping Maneuver:
When you hit a creature with a melee weapon attack, you can expend one superiority die to attempt to damage another creature with the same attack. Choose another creature within 5 feet of the original target and within your reach. If the original attack roll would hit the second creature, it takes damage equal to the number you roll on your superiority die. The damage is of the same type dealt by the original attack.
Here a single attack is targeting multiple creatures and thus it could be used while under the effects of slow.
There is also the green-flame blade spell:
As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range, otherwise the spell fails. On a hit, the target suffers the attack's normal effects, and green fire leaps from the target to a different creature of your choice that you can see within 5 feet of it. The second creature takes fire damage equal to your spellcasting ability modifier. This spell's damage increases when you reach higher levels.
And there is also the ice knife spell as well.
Are there other examples of things like this, where a single attack damages multiple creatures? Note I do not care about situational things such as "if the enemy has warding bond I would damage two creatures". I want these to be options available to the attacker, dependant only up to where the enemies are (and also their type if that even matters)