his home treated him as honorable and other cultures don’t

Not the point of the story, when NPCs get to know who the character is theirs opinion changes

it wasn’t because they mistook him for being the kind of person you get to do that to

That is actually almost what happened. If he was not a mage, that story point would change littls.

The “mages are slaves” thing is more akin to FF6’s “there is no magic in this world”, like it is a somewhat big deal that Terra is mage, but game doesn’t spend much time there since it is not a point.

Your character is not a slave. (Spoilers limited to promotional materials) Player character is the oldest son of the ruler of one of the major countries in the game world, so a prince. Ability to wield (very specific) magic is quickly explained that some of the nobles of that family can do. He somewhat is a slave at some point, but this is a very brief story moment (tbf at the very beginning, you meet your character as a slave before he goes into childhood memory where he is a prince). When relevant, NPCs do interact with character as with slave, but its rarely relevant. So it is very much a background theme, even if a major one.