(...)When he tells f!Lavellan about the vallaslin, he does not say anything about those who served in the temples of the Creators, which we eventually have to learn about on our own simply because Solas doesn't want to say anything about it. It's his way to lie via omission, which happens pretty much the entire game.
It seems to actually be a recurring theme of tales concerning Dread Wolf, distorted or not. Even in the "Fen'Harel betrayed all the gods" the wolf doesn't explicitly lie:
- he promises the Creators that he would end the war
- he promises the Forgotten Ones that he will arrange for Creators' defeat
End result? War is over: check. Creators are defeated: check.
Either way, about Vallaslin - I find it perfectly plausible that temple's sentinels were recruited from slaves of nobles favoring Mythal. W know little about slavery in Arlathan, what's certain is only what comes from the very definition of slave: they were not their own people. That's all. Imagine now those slaves, marked at some point so that their very existance honors Mythal (or some other god, but let's talk Mythal here). They're still noble's slaves at this point. But then the noble hand-picks the best, most faithful and most deserving of them and sends them to temple to become servants of Mythal - isn't that a worthy offering? But that doesn't mean the slaves don't want this, or aren't priests. It could even very well be a case where slaves are literally the only ones considered qualified to become proper priests, as being raised as another's property supposedly makes them more selfless, without personal urges that would stand in the way of their duty. They're not - and were never - their own people, so they can become the voice of a god without personal concerns clouding their judgement.
That's, of course, just one of many possible interpretations, but what I wanted to show here is: we know *** about Arlathan and its culture. There were slaves, but when we think "slaves" we imagine those people working plantations or dying in poorly-ventilated mines. Historically slaves had many functions, including teachers and scribes. In-world let's take a look at Qunari - when you think about it, they're a society that in practice consists 100% of slaves. Everyone has their job, way of life, mating partners and whatever you name it assigned to them. There is no personal freedom, they don't even have their names. They have warrior slaves, scientist slaves, ruling slaves, priest-slaves.
Basically, slaves can be many different things and just the fact "they're slaves" tells us very little about them without context, namely: what are slaves in this particular culture.
About Arlathan, we can only speculate, even if we take what Solas says at face value, assuming that vallaslin=slave markings and therefore sentinels are literally slaves. This still doesn't tell us much...