[RPG] Does the Sharpshooter feat work on ranged spells that require an attack roll

War Caster lets you cast a spell instead of an opportunity attack, which is made just before the creature leaves your reach. If your reach is 5 feet and you use War Caster to cast a ranged spell attack then, yes, the attack is made with disadvantage.

If you have a natural reach greater than 5 feet, then the attack is made when the creature is more than 5 feet away, so this doesn't apply. Similarly, it doesn't apply to melee spell attacks or spells that use saving throws. You are also good if you have the Crossbow Expert feat.

No, you can't take the -5/+10 while throwing a dagger.

Thrown: If a weapon has the thrown property, you can throw the weapon to make a ranged attack.

Throwing a dagger is a ranged weapon attack, so the first two benefits of the Sharpshooter feat will apply. However, the third benefit doesn't require a ranged weapon attack:

• Before you make an attack with a ranged weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add +10 to the attack’s damage.

It specifically requires that you attack with a ranged weapon. The thrown property doesn't change the fact that a dagger is a melee weapon. Throwing a dart, for example, would benefit from the -5/+10, because it's a ranged weapon. The thrown property isn't what's important here - what's important is whether the weapon is a ranged weapon or not.

If you prefer to hear it from the designers, Jeremy Crafowrd agrees that the first two benefits of Sharpshooter work with daggers, but not the 3rd. He also stated that a dagger is a melee weapon, even when you throw it, as well as including it in the official Sage Advice Compendium:

Does the Archery fighting style work with a melee weapon that you throw? No, the Archery feature benefits ranged weapons. A melee weapon, such as a dagger or handaxe, is still a melee weapon when you make a ranged attack with it.