For physical attacks, the scenario where you don't add your Proficiency bonus to an attack roll is relatively clear-cut: when you don't have proficiency in a weapon. If you're a Sorcerer trying to wield a Rapier, with a DEX bonus of +3, then your Attack Roll is +3; no Proficiency added.
However, for spellcasters, I can't find a single scenario where the Proficiency Bonus does not get added to a spell, either to the Attack Roll or the corresponding Saving Throw DC, except, of course, for spells which don't have an Attack Roll or Saving Throw.
Obviously, I respect the balancing implications of this choice: if, for example, things like Racial feature spells/cantrips didn't scale with a character's Proficiency bonus, they would end up being very weak as a character leveled up, whereas with the proficiency bonus, a Level 20 Fighter casting their racial spell Burning Hands can at least expect to challenge the Saving Throw capabilities of their targets, even if the damage is pretty pitiful relative to the kinds of creatures that would pose a meaningful challenge at their level.
But it is strange to me that there appears to be an entire class of features in the game where there's no variance as to whether a character's Proficiency Bonus ought to be added, or not. So it begs the question:
Does there exist some corner-case scenario in 5th Edition D&D where a character would not add their Proficiency Bonus to the Attack Roll or Saving Throw DC of a spell they cast? Or does such a scenario simply not exist?
Best Answer
I can't provide evidence to confirm a negative, but in my experience (thousands of hours of DMing and playing), such a scenario does not exist. If a character is casting a spell personally rather than via an item, they will use their proficiency bonus. Some items - scrolls and wands are the most common - have a fixed DC; the character activating the item has no effect.
By contrast, lacking the other half of that equation can exist. A Thief Rogue with Use Magic Device does not have a spellcasting ability, and therefore uses +0 for magic items that may require one.
From the Basic Rules, Chapter 14, Activating an Item, Spells: