It is invented for Baldur's Gate 3
There is no ability called Healing Radiance in published 5e D&D - I certainly don't remember reading about it, and a text search on D&D Beyond should turn up such a reference if one existed in any sourcebook's text. Larian has invented it for BG3, as they have made many significant changes to the game's rules and class progressions in the adaption of 5e.
Specifically for paladins, you might notice that Larian has shifted the choice of a paladin's oath from a 3rd level feature to a 1st level one, and in order to make that mechanically meaningful added a new 1st level feature to each oath that doesn't exist in 5e; Ancients paladins get Healing Radiance, Devotion paladins get Holy Rebuke, and Vengeance paladins get Inquisitor's Might.
5e alternatives
The nearest thing to BG3's ability which I can think of is the Life domain cleric's Channel Divinity option Preserve Life:
As an action, you present your holy symbol and evoke healing energy that can restore a number of hit points equal to five times your cleric level. Choose any creatures within 30 feet of you, and divide those hit points among them. This feature can restore a creature to no more than half of its hit point maximum. You can’t use this feature on an undead or a construct.
The BG3 ability could be considered a simplified version of this (and the version that is actually implemented for Life clerics in BG3 is a different, simpler version of this). But obviously, this is not an option actually available for paladins.
For paladins, I think the closest equivalent is the 3rd level spell aura of vitality:
Healing energy radiates from you in an aura with a 30-foot radius. Until the spell ends, the aura moves with you, centered on you. You can use a bonus action to cause one creature in the aura (including you) to regain 2d6 hit points.
This isn't a general AoE heal, but as healing spells go it's very efficient per spell slot (getting 20d6 of healing out of one level 3 slot if maximally used) and gives the paladin a way to use their often superfluous bonus action. Paladins are the only class that has access to it by default and it's only available for a couple of subclasses of other classes, so in this sense it is sort of a paladin feature. However, it is one you gain very late, as you need to be 9th level to have 3rd level slots.
