...And six years after release WOTC still does not fix it (like the lucky feat, ranger class or the abjurer-warlock with the cloak of shadows, The fact that you can use action surge with spells, etc) so I think this is intended or they do not fix it because it could create a backlash.
What makes Divine Smite too strong is that you can choose to use it AFTER the attack roll. Most abilities that trigger on attack rolls have to be used before and this was never updated.
And also you can use it many times as long as you have spells available to continue smiting in the same round. Other features, like rogue´s sneak or monks`furry of blows you can only use once per round
(I.E: a Paladin at 9th level attacking with a longsword can do a total of 10d8 damage (+ str modifier doubled) against a baddie in one round with no save (ie. two attacks at 1d8 [longsword] plus 4d8 [3rd level spell slot] each). If his opponent is undead or devilish that goes up to 12d8 total per round When the Paladin gets improved Divine Smite at 11th level he could deal 14d8 damage in one round to an undead/devil foe– and who hasn’t fought in campaigns where many foes are under that descriptor like Curse of Stradh, Descent into Avernus or Tomb of annihilation?)
Yes, the paladin is able to give a decent and consistent damage output it still requires the right scenario: melee & and spellslots.
The rogue can do sneak attack every round given the right circumstances...forever. at both melee and range. Also, the rogue can hide after each attack using cunning action. On top of that the rogue receives 3 feats at 4th, 8th, 10th level. Further more the rogue can halve damage using uncanny Dodge, and evasion. Last but not least the rogue receives two expertise which doubles the proficiency bonus during level 1-10.
An wizard can do plenty of damage, using a fireball and plenty of enemies with in a small area. On top of that the wizard can cast hold person which essentially is a 1st level power word kill. Once held they are dead. And last but not least, magic missile as evocation has and insane damage output with no chance of saving or mitigating it. Unless you happen to be a wizard.
What's the point of this? Simply put: you cannot compare damage output cross class as it depends on the scenario and the resources available at the time.
Stating that paladins are OP is a ridiculous statement. It depends on the scenario and the point of view. Smite is a part of the class and should not be meddled with.
Just to be clear I have played: warlock, wizard, paladins, ranger in DND5e in pen and paper. I have no problem with any of them except perhaps the ranger with is rather bland.
