This may be true, but it does invite a question: what provides the definition of "good" in D&D terms? What's the authoritative text(s)?
For (relevant) example, the 5e rules are pretty clear on the subject of warlocks & alignment. A warlock can be Good. Even if they have a Fiend patron. The text isn't ambiguous.
Naturally - I'm talking about a Celestial patron, however. That is totally different. A warlock can even worship Pelor, Loviatar, the Silver Flame, Bahamut, whatever, and still have a fiend, archfey, or abomination for a Patron.
But having an archon or angelic being for a patron is an entirely different matter.
What is the authoritative text? Right now, its the PHB, which gives definitions of each alignment, as well as descriptions of gods and how they divide domains.
Certainly! But you asked what forbidden lore would look like and what angelic creatures would sanction such "dark"-seeming powers, and so I felt it reasonable to point out that, as far as the archetype/character inspiration/etc. goes, there's plenty of warlock-y stuff in gods of goodness, life, and light. A player who wants a more "Ezekiel"-style character or one who sees themselves as a bringer of plagues and divine judgement, could certainly find a home in a hypothetical Celestial-pact Warlock.
Except that "bringer of plagues" in D&D terms is an evil god, not a good one. In 5e, it would be a god with the Death Domain. Talona, goddess of disease and poison from Faerun is Chaotic Evil. Incabulos, god of plague and famine of Greyhawk is Neutral Evil. Morgion, god of disease and secrecy, is a Neutral Evil god of Dragonlance.
Your real world examples simply don't work because Pelor and the other Lawful and Neutral Good deities aren't the Christian God.
I don't know that this is true. Gods in D&D are NPC's. Alignment in D&D isn't monolithic. The hidden secrets of Pelor may bring darkness and plague and undead upon his enemies ("If you do not pay homage to the sun, the sun will deprive you of its light, and darkness will reign. I am that darkness!"), in the name of Good or at least in the name of anti-Evil.
That's the portfolio of a different god. Trying to put that under Pelor is a major stretch at the least. If you don't worship Pelor, he'll weaken and other, dark gods will rise to prominence is the usual threat in D&D. Pelor being the one to bring the plagues is out of character for him.
Now, playing a character who's made a pact with dark forces to scare people into worshiping Pelor? That makes sense for some people. I've seen stranger. Its not the same thing as a D&D Solar Angel being the teacher.
It's possible for Bahamut to over-reach and do something questionable in the name of righteousness.
Good people make mistakes. Especially in the heat of passion. Teaching warlocks is a very deliberate and premeditated choice, and giving out spells that go against much of what the metalic dragons believe in and stand for is highly questionable when there's no need of it - Bahamut does have Vengence Oath Paladins for extreme cases.
I think there is no more appropriate weapon for a god of healing and life to use than pestilence and death. When Pelor is angry with you, you get sick in the darkness and die. When Pelor is angry with you, the Celestial Warlock might come to visit you. They are proclaimed as heretics and liars.
When Pelor is angry with you, that's when you get struck down by light from the heavens in the form of Light Clerics, Paladins, and radiant angel servitors.
You seem to be missing something rather fundamental here. Pelor has an established history, personality, dogma, and portfolio. You are trying to twist him into something he's not in order to fit your theory. Its not working.
I mean, Apollo is a brilliant example as well. Light and life and delight and when he's offended you get pestilential arrows.
Again, real life gods do not equate to D&D gods. That's a false dichotomy.
Warlocks with a Celestial pact can explore that aspect of the gods of goodness and light in play.
"That aspect of goodness and light which the Greyhawk setting firmly establishes as the domain of one of Pelor's evil god rivals."
Warlocks with a Celstial pact should explore the redemption path, just as an Oathbreaker follows the paladin who's given up his sacred vows for a dark desire born of hate or lust or greed.
Not to say that this is the way it must be, of course, just to say that this is an interesting option that isn't logically dismissible. You can say you're not interested in it, or that it doesn't fit with the way you envision the gods in your games, but it's a harder leg to stand on to say that it is something that is inconceivable or nonsensical or is incompatible with D&D as a whole.
Its incompatable with the established D&D worlds, such as Pelor and the gods of Greyhawk. Or his 4e incarnation in Nentir Vale. It doesn't fit with what we know of the celestial creatures in the 5e monster manual.
You are free to houserule things the way you want in private games. However, trying to equate Good aligned gods with those that offer threat if you don't serve them doesn't fit within the descriptions of alignment as we know it. Alignment is flexible, yes, to an extent. That extent does not extend to a NG acting like a tyrant, killing those who don't follow them; tyranny is firmly established towards the LE end of the alignment chart.
If you want to play the devil's advocate, that's fine. But simply ignoring things long established makes no sense what so ever. Rewriting Pelor until he's no longer Pelor isn't the answer.