We look so hard for a black and white to support or position in everything but the fact is there are few if any black and whites
To say “religion = slavery” is illogical.
To say no religion turned a blind eye to it and or did not encourage it is also illogical.
To say “religion” this or that is also illogical. Many religions, mostly God based did of course condone and encourage or at least turn a blind eye to it, but not all.
T'ai Chi my friend to say that Judeo-Christianity on a whole today does not condone and encourage slavery would be greatly correct.
To say it did not in the past or that the teachings or the NT and OT did not teach and so condone it would be dishonest.
The OT of course condones many passages doing this as does the NT. Mohammed also owned and condoned beating slaves.
I may be wrong but believe you may be leaning as to this conversation towards the Christian belief.
Remember of the many OT passages concerning this EX 21:20,21 God clearly says we can beat slaves to death.
Exodus 21
20 And if a man smite his servant, or his maid, with a rod, and he die under his hand; he shall be surely punished.
21 Notwithstanding, if he continue a day or two, he shall not be punished: for he is his money.
As to Jesus
Luke 12
47 And that servant, which knew his lord's will, and prepared not himself, neither did according to his will, shall be beaten with many stripes.
48 But he that knew not, and did commit things worthy of stripes, shall be beaten with few stripes. For unto whomsoever much is given, of him shall be much required: and to whom men have committed much, of him they will ask the more.
This of course is not in line with the great quote at the bottom of your post.
The soft overcomes the hard;
the gentle overcomes the rigid.
Violence has a habit of returning.
Your point that “religion = slavery” is wrong is true when said in any sweeping fashion as to say religion if free or has clean hands as to it is also wrong when said in a sweeping fashion.
To say the National Geographic's Slavery article points to today and does not mention some religions means religion has no hand in it at all is not a clear look at the roots of this tree.
Of course as to that religion is not the main root of slavery, not any religion greed is but to say some religions thousands of years ago did not facilitate in their teachings slavery is not the case.
These are old religions during an old time things have gotten better and while the “back then it was different” is often used remember some, a few very old religions were very much against slavery.
One last thought, if one in a God based religion were to say “back then it was different, that is why they did it then and now we don’t” can they really say their God is always right, unchanging etc?
What is wrong in a “perfect mind” as to an act that brings harm is always wrong.
That is just what I believe.